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### Posix Time

## Posix Time System

Introduction -- Usage Examples

### Introduction

Defines a non-adjusted time system with nano-second/micro-second resolution and stable calculation properties. The nano-second resolution option uses 96 bits of underlying storage for each ptime while the micro-second resolution uses 64 bits per ptime (see Build Options for details). This time system uses the Gregorian calendar to implement the date portion of the time representation.

### Usage Examples

Example | Description
--- | ---
Time Math | A few simple calculations using ptime and time_durations.
Print Hours | Retrieve time from clock, use a time_iterator.
Local to UTC Conversion | Demonstrates a couple different ways to convert a local to UTC time including daylight savings rules.
Time Periods | Some simple examples of intersection and display of time periods.

#### Ptime

Introduction -- Header -- Construction -- Construct from String -- Construct from Clock -- Construct using Conversion functions -- Accessors -- Conversion To String -- Operators -- Struct tm, time_t, and FILETIME Functions

### Introduction

The class boost::posix_time::ptime is the primary interface for time point manipulation. In general, the ptime class is immutable once constructed although it does allow assignment.

Class ptime is dependent on gregorian::date for the interface to the date portion of a time point.

Other techniques for creating times include time iterators.

### Header

    #include "boost/date_time/posix_time/posix_time.hpp" //include all types plus i/o
    or
    #include "boost/date_time/posix_time/posix_time_types.hpp" //no i/o just types

### Construction

Syntax | Description
--- | ---
Example
    ptime(date,time_duration)
| Construct from a date and offset
    ptime t1(date(2002,Jan,10),
             time_duration(1,2,3));
    ptime t2(date(2002,Jan,10),
             hours(1)+nanosec(5));
    ptime(ptime)
| Copy constructor
    ptime t3(t1)
    ptime(special_values sv)
| Constructor for infinities, not-a-date-time, max_date_time, and min_date_time
    ptime d1(neg_infin);
    ptime d2(pos_infin);
    ptime d3(not_a_date_time);
    ptime d4(max_date_time);
    ptime d5(min_date_time);
    ptime;
| Default constructor. Creates a ptime object initialized to not_a_date_time. NOTE: this constructor can be disabled by defining DATE_TIME_NO_DEFAULT_CONSTRUCTOR (see compiler_config.hpp)
    ptime p; // p => not_a_date_time

### Construct from String

Syntax | Description
--- | ---
Example
    ptime time_from_string(std::string)
| From delimited string. NOTE: Excess digits in fractional seconds will be dropped. Ex: "1:02:03.123456999" => 1:02:03.123456. This behavior is affected by the precision the library is compiled with (see Build-Compiler Information.
    std::string ts("2002-01-20 23:59:59.000");
    ptime t(time_from_string(ts))
    ptime from_iso_string(std::string)
| From non delimited iso form string.
    std::string ts("20020131T235959");
    ptime t(from_iso_string(ts))

### Construct from Clock

Syntax | Description
--- | ---
Example
    ptime second_clock::local_time()
| Get the local time, second level resolution, based on the time zone settings of the computer.
    ptime t(second_clock::local_time());
    ptime second_clock::universal_time()
| Get the UTC time.
    ptime t(second_clock::universal_time())
    ptime microsec_clock::local_time()
| Get the local time using a sub second resolution clock. On Unix systems this is implemented using GetTimeOfDay. On most Win32 platforms it is implemented using ftime. Win32 systems often do not achieve microsecond resolution via this API. If higher resolution is critical to your application test your platform to see the achieved resolution.
    ptime t(microsec_clock::local_time());
    ptime microsec_clock::universal_time()
| Get the UTC time using a sub second resolution clock. On Unix systems this is implemented using GetTimeOfDay. On most Win32 platforms it is implemented using ftime. Win32 systems often do not achieve microsecond resolution via this API. If higher resolution is critical to your application test your platform to see the achieved resolution.
    ptime t(microsec_clock::universal_time());

### Construct using Conversion Functions

Syntax | Description
--- | ---
Example
    ptime from_time_t(time_t t);
| Converts a time_t into a ptime.
    ptime t = from_time_t(tt);
    ptime from_ftime<ptime>(FILETIME ft);
| Creates a ptime object from a FILETIME structure.
    ptime t = from_ftime<ptime>(ft);

### Accessors

Syntax | Description
--- | ---
Example
    date date()
| Get the date part of a time.
    date d(2002,Jan,10);
    ptime t(d, hour(1));
    t.date() --> 2002-Jan-10;
    time_duration time_of_day()
| Get the time offset in the day.
    date d(2002,Jan,10);
    ptime t(d, hour(1));
    t.time_of_day() --> 01:00:00;
    bool is_infinity() const
| Returns true if ptime is either positive or negative infinity
    ptime pt(pos_infin);
    pt.is_infinity(); // --> true
    bool is_neg_infinity() const
| Returns true if ptime is negative infinity
    ptime pt(neg_infin);
    pt.is_neg_infinity(); // --> true
    bool is_pos_infinity() const
| Returns true if ptime is positive infinity
    ptime pt(neg_infin);
    pt.is_pos_infinity(); // --> true
    bool is_not_a_date_time() const
| Returns true if value is not a ptime
    ptime pt(not_a_date_time);
    pt.is_not_a_date_time(); // --> true
    bool is_special() const
| Returns true if ptime is any `special_value`
    ptime pt(pos_infin);
    ptime pt2(not_a_date_time);
    ptime pt3(date(2005,Mar,1), hours(10));
    pt.is_special(); // --> true
    pt2.is_special(); // --> true
    pt3.is_special(); // --> false

### Conversion to String

Syntax | Description
--- | ---
Example
    std::string to_simple_string(ptime)
| To `YYYY-mmm-DD HH:MM:SS.fffffffff` string where `mmm` 3 char month name. Fractional seconds only included if non-zero.
    2002-Jan-01 10:00:01.123456789
    std::string to_iso_string(ptime)
| Convert to form `YYYYMMDDTHHMMSS,fffffffff` where `T` is the date-time separator
    20020131T100001,123456789
    std::string to_iso_extended_string(ptime)
| Convert to form `YYYY-MM-DDTHH:MM:SS,fffffffff` where `T` is the date-time separator
    2002-01-31T10:00:01,123456789

### Operators

Syntax | Description
--- | ---
Example
    operator<<, operator>>
| Streaming operators. Note: As of version 1.33, streaming operations have been greatly improved. See Date Time IO System for more details (including exceptions and error conditions).
    ptime pt(not_a_date_time);
    stringstream ss("2002-Jan-01 14:23:11");
    ss >> pt;
    std::cout << pt; // "2002-Jan-01 14:23:11"
    operator==, operator!=,
    operator>, operator<,
    operator>=, operator<=
| A full complement of comparison operators
    t1 == t2, etc
    ptime operator+(days)
| Return a ptime adding a day offset
    date d(2002,Jan,1);
    ptime t(d,minutes(5));
    days dd(1);
    ptime t2 = t + dd;
    ptime operator-(days)
| Return a ptime subtracting a day offset
    date d(2002,Jan,1);
    ptime t(d,minutes(5));
    days dd(1);
    ptime t2 = t - dd;
    ptime operator+(time_duration)
| Return a ptime adding a time duration
    date d(2002,Jan,1);
    ptime t(d,minutes(5));
    ptime t2 = t + hours(1) + minutes(2);
    ptime operator-(time_duration)
| Return a ptime subtracting a time duration
    date d(2002,Jan,1);
    ptime t(d,minutes(5));
    ptime t2 = t - minutes(2);
    time_duration operator-(ptime)
| Take the difference between two times.
    date d(2002,Jan,1);
    ptime t1(d,minutes(5));
    ptime t2(d,seconds(5));
    time_duration t3 = t2 - t1;//negative result

### Struct tm, time_t, and FILETIME Functions

Functions for converting posix_time objects to, and from, `tm` structs are provided as well as conversion from `time_t` and `FILETIME`.

Syntax | Description
--- | ---
Example
    tm to_tm(ptime)
| A function for converting a `ptime` object to a `tm` struct. The `tm_isdst` field is set to -1.
    ptime pt(date(2005,Jan,1), time_duration(1,2,3));
    tm pt_tm = to_tm(pt);
    /* tm_year => 105
       tm_mon  => 0
       tm_mday => 1
       tm_wday => 6 (Saturday)
       tm_yday => 0
       tm_hour => 1
       tm_min  => 2
       tm_sec  => 3
       tm_isddst => -1 */
    date date_from_tm(tm datetm)
| A function for converting a `tm` struct to a `date` object. The fields: `tm_wday` , `tm_yday` , and `tm_isdst` are ignored.
    tm pt_tm;
    pt_tm.tm_year = 105;
    pt_tm.tm_mon  = 0;
    pt_tm.tm_mday = 1;
    pt_tm.tm_hour = 1;
    pt_tm.tm_min  = 2;
    pt_tm.tm_sec  = 3;
    ptime pt = ptime_from_tm(pt_tm);
    // pt => 2005-Jan-01 01:02:03
    tm to_tm(time_duration)
| A function for converting a `time_duration` object to a `tm` struct. The fields: `tm_year`, `tm_mon`, `tm_mday`, `tm_wday`, `tm_yday` are set to zero. The `tm_isdst` field is set to -1.
    time_duration td(1,2,3);
    tm td_tm = to_tm(td);
    /* tm_year => 0
       tm_mon  => 0
       tm_mday => 0
       tm_wday => 0
       tm_yday => 0
       tm_hour => 1
       tm_min  => 2
       tm_sec  => 3
       tm_isddst => -1 */
    ptime from_time_t(std::time_t)
| Creates a `ptime` from the time_t parameter. The seconds held in the time_t are added to a time point of 1970-Jan-01.
    ptime pt(not_a_date_time);
    std::time_t t;
    t = 1118158776;
    pt = from_time_t(t);
    // pt => 2005-Jun-07 15:39:36
    ptime from_ftime<ptime>(FILETIME)
| A template function that constructs a `ptime` from a FILETIME struct.
    FILETIME ft;
    ft.dwHighDateTime = 29715317;
    ft.dwLowDateTime = 3865122988UL;
    ptime pt = from_ftime<ptime>(ft);
    // pt => 2005-Jun-07 15:30:57.039582000

#### Time Duration

Introduction -- Header -- Construction -- Count Based Construction -- Construct from String -- Accessors -- Conversion To String -- Operators -- Struct tm Functions

### Introduction

The class boost::posix_time::time_duration the base type responsible for representing a length of time. A duration can be either positive or negative. The general time_duration class provides a constructor that takes a count of the number of hours, minutes, seconds, and fractional seconds count as shown in the code fragment below. The resolution of the time_duration is configure able at compile time. See Build-Compiler Information for more information.

    using namespace boost::posix_time;
    time_duration td(1,2,3,4); //01:02:03.000000004 when resolution is nano seconds
    time_duration td(1,2,3,4); //01:02:03.000004 when resolution is micro seconds

Several small helper classes that derive from a base time_duration, as shown below, to adjust for different resolutions. These classes can shorten code and make the intent clearer.

As an example:

    using namespace boost::posix_time;

    time_duration td = hours(1) + seconds(10); //01:00:01
    td = hours(1) + nanoseconds(5); //01:00:00.000000005

Note that the existence of the higher resolution classes (eg: nanoseconds) depends on the installation of the library. See Build-Compiler Information for more information.

Another way to handle this is to utilize the ticks_per_second() method of time_duration to write code that is portable no matter how the library is compiled. The general equation for calculating a resolution independent count is as follows:

    count*(time_duration_ticks_per_second / count_ticks_per_second)

For example, let's suppose we want to construct using a count that represents tenths of a second. That is, each tick is 0.1 second.

    int number_of_tenths = 5;
    //create a resolution independent count -- divide by 10 since there are
    //10 tenths in a second.
    int count = number_of_tenths*(time_duration::ticks_per_second()/10);
    time_duration td(1,2,3,count); //01:02:03.5 //no matter the resolution settings

### Header

    #include "boost/date_time/posix_time/posix_time.hpp" //include all types plus i/o
    or
    #include "boost/date_time/posix_time/posix_time_types.hpp" //no i/o just types

### Construction

Syntax | Description
--- | ---
Example
    time_duration(hours,
                  minutes,
                  seconds,
                  fractional_seconds)
| Construct a duration from the counts. The fractional_second parameter is a number of units and is therefore affected by the resolution the application is compiled with (see Build-Compiler Information). If the fractional_seconds argument exceeds the limit of the compiled precision, the excess value will be "carried over" into the seconds field. See above for techniques to creating a resolution independent count.
    time_duration td(1,2,3,9);
    //1 hr 2 min 3 sec 9 nanoseconds
    time_duration td2(1,2,3,123456789);
    time_duration td3(1,2,3,1000);
    // with microsecond resolution (6 digits)
    // td2 => "01:04:06.456789"
    // td3 => "01:02:03.001000"
    // with nanosecond resolution (9 digits)
    // td2 => "01:02:03.123456789"
    // td3 => "01:02:03.000001000"
    time_duration(special_value sv)
| Special values constructor. Important note: When a time_duration is a special value, either by construction or other means, the following accessor functions will give unpredictable results:
    hours(), minutes(), seconds(), ticks(),
    fractional_seconds(), total_nanoseconds(),
    total_microseconds(), total_milliseconds(),
    total_seconds()
The remaining accessor functions will work as expected.

### Count Based Construction

Syntax | Description
--- | ---
Example
    hours(long)
| Number of hours
    time_duration td = hours(3);
    minutes(long)
| Number of minutes
    time_duration td = minutes(3);
    seconds(long)
| Number of seconds
    time_duration td = seconds(3);
    milliseconds(long)
| Number of milliseconds.
    time_duration td = milliseconds(3);
    microseconds(long)
| Number of microseconds.
    time_duration td = microseconds(3);
    nanoseconds(long)
| Number of nanoseconds.
    time_duration td = nanoseconds(3);

### Construct from String

Syntax | Description
--- | ---
Example
    time_duration duration_from_string(std::string)
| From delimited string. NOTE: Excess digits in fractional seconds will be dropped. Ex: "1:02:03.123456999" => 1:02:03.123456. This behavior is affected by the precision the library is compiled with (see Build-Compiler Information.
    std::string ts("23:59:59.000");
    time_duration td(duration_from_string(ts));

### Accessors

Syntax | Description
--- | ---
Example
    long hours()
| Get the number of normalized hours (will give unpredictable results if calling `time_duration` is a `special_value`).
    time_duration td(1,2,3);
    time_duration neg_td(-1,2,3);
    td.hours(); // --> 1
    neg_td.hours(); // --> -1
    long minutes()
| Get the number of minutes normalized +/-(0..59) (will give unpredictable results if calling `time_duration` is a `special_value`).
    time_duration td(1,2,3);
    time_duration neg_td(-1,2,3);
    td.minutes(); // --> 2
    neg_td.minutes(); // --> -2
    long seconds()
| Get the normalized number of second +/-(0..59) (will give unpredictable results if calling `time_duration` is a `special_value`).
    time_duration td(1,2,3);
    time_duration neg_td(-1,2,3);
    td.seconds(); // --> 3
    neg_td.seconds(); // --> -3
    long total_seconds()
| Get the total number of seconds truncating any fractional seconds (will give unpredictable results if calling `time_duration` is a `special_value`).
    time_duration td(1,2,3,10);
    td.total_seconds();
    // --> (1*3600) + (2*60) + 3 == 3723
    long total_milliseconds()
| Get the total number of milliseconds truncating any remaining digits (will give unpredictable results if calling `time_duration` is a `special_value`).
    time_duration td(1,2,3,123456789);
    td.total_milliseconds();
    // HMS --> (1*3600) + (2*60) + 3 == 3723 seconds
    // milliseconds is 3 decimal places
    // (3723 * 1000) + 123 == 3723123
    long total_microseconds()
| Get the total number of microseconds truncating any remaining digits (will give unpredictable results if calling `time_duration` is a `special_value`).
    time_duration td(1,2,3,123456789);
    td.total_microseconds();
    // HMS --> (1*3600) + (2*60) + 3 == 3723 seconds
    // microseconds is 6 decimal places
    // (3723 * 1000000) + 123456 == 3723123456
    long total_nanoseconds()
| Get the total number of nanoseconds truncating any remaining digits (will give unpredictable results if calling `time_duration` is a `special_value`).
    time_duration td(1,2,3,123456789);
    td.total_nanoseconds();
    // HMS --> (1*3600) + (2*60) + 3 == 3723 seconds
    // nanoseconds is 9 decimal places
    // (3723 * 1000000000) + 123456789
    //                              == 3723123456789
    long fractional_seconds()
| Get the number of fractional seconds (will give unpredictable results if calling `time_duration` is a `special_value`).
    time_duration td(1,2,3, 1000);
    td.fractional_seconds(); // --> 1000
    bool is_negative()
| True if duration is negative.
    time_duration td(-1,0,0);
    td.is_negative(); // --> true
    time_duration invert_sign()
| Generate a new duration with the sign inverted/
    time_duration td(-1,0,0);
    td.invert_sign(); // --> 01:00:00
    date_time::time_resolutions resolution()
| Describes the resolution capability of the time_duration class. time_resolutions is an enum of resolution possibilities ranging from seconds to nanoseconds.
    time_duration::resolution() --> nano
    time_duration::num_fractional_digits()
| Returns an unsigned short holding the number of fractional digits the time resolution has.
    unsigned short secs;
    secs = time_duration::num_fractional_digits();
    // 9 for nano, 6 for micro, etc.
    time_duration::ticks_per_second()
| Return the number of ticks in a second. For example, if the duration supports nanoseconds then the returned result will be 1000000.
    std::cout << time_duration::ticks_per_second();
    boost::int64_t ticks()
| Return the raw count of the duration type (will give unpredictable results if calling `time_duration` is a `special_value`).
    time_duration td(0,0,0, 1000);
    td.ticks() // --> 1000
    time_duration unit()
| Return smallest possible unit of duration type (1 nanosecond).
    time_duration::unit() --> time_duration(0,0,0,1)
    bool is_neg_infinity() const
| Returns true if time_duration is negative infinity
    time_duration td(neg_infin);
    td.is_neg_infinity(); // --> true
    bool is_pos_infinity() const
| Returns true if time_duration is positive infinity
    time_duration td(neg_infin);
    td.is_pos_infinity(); // --> true
    bool is_not_a_date_time() const
| Returns true if value is not a time
    time_duration td(not_a_date_time);
    td.is_not_a_date_time(); // --> true
    bool is_special() const
| Returns true if time_duration is any `special_value`
    time_duration td(pos_infin);
    time_duration td2(not_a_date_time);
    time_duration td3(2,5,10);
    td.is_special(); // --> true
    td2.is_special(); // --> true
    td3.is_special(); // --> false

### Conversion To String

Syntax | Description
--- | ---
Example
    std::string to_simple_string(time_duration)
| To `HH:MM:SS.fffffffff` were `fff` is fractional seconds that are only included if non-zero.
    10:00:01.123456789
    std::string to_iso_string(time_duration)
| Convert to form `HHMMSS,fffffffff`.
    100001,123456789

### Operators

Syntax | Description
--- | ---
Example
    operator<<, operator>>
| Streaming operators. Note: As of version 1.33, streaming operations have been greatly improved. See Date Time IO System for more details (including exceptions and error conditions).
    time_duration td(0,0,0);
    stringstream ss("14:23:11.345678");
    ss >> td;
    std::cout << td; // "14:23:11.345678"
    operator==, operator!=,
    operator>, operator<,
    operator>=, operator<=
| A full complement of comparison operators
    dd1 == dd2, etc
    time_duration operator+(time_duration)
| Add durations.
    time_duration td1(hours(1)+minutes(2));
    time_duration td2(seconds(10));
    time_duration td3 = td1 + td2;
    time_duration operator-(time_duration)
| Subtract durations.
    time_duration td1(hours(1)+nanoseconds(2));
    time_duration td2 = td1 - minutes(1);
    time_duration operator/(int)
| Divide the length of a duration by an integer value. Discards any remainder.
    hours(3)/2 == time_duration(1,30,0);
    nanosecond(3)/2 == nanosecond(1);
    time_duration operator*(int)
| Multiply the length of a duration by an integer value.
    hours(3)*2 == hours(6);

### Struct tm, time_t, and FILETIME Functions

Function for converting a time_duration to a `tm` struct is provided.

Syntax | Description
--- | ---
Example
    tm to_tm(time_duration)
| A function for converting a `time_duration` object to a `tm` struct. The fields: `tm_year`, `tm_mon`, `tm_mday`, `tm_wday`, `tm_yday` are set to zero. The `tm_isdst` field is set to -1.
    time_duration td(1,2,3);
    tm td_tm = to_tm(td);
    /* tm_year => 0
       tm_mon  => 0
       tm_mday => 0
       tm_wday => 0
       tm_yday => 0
       tm_hour => 1
       tm_min  => 2
       tm_sec  => 3
       tm_isddst => -1 */

#### Time Period

Introduction -- Header -- Construction -- Accessors -- Conversion To String -- Operators

### Introduction

The class boost::posix_time::time_period provides direct representation for ranges between two times. Periods provide the ability to simplify some types of calculations by simplifying the conditional logic of the program.

A period that is created with beginning and end points being equal, or with a duration of zero, is known as a zero length period. Zero length periods are considered invalid (it is perfectly legal to construct an invalid period). For these periods, the `last` point will always be one unit less that the `begin` point.

The time periods example provides an example of using time periods.

### Header

    #include "boost/date_time/posix_time/posix_time.hpp" //include all types plus i/o
    or
    #include "boost/date_time/posix_time/posix_time_types.hpp" //no i/o just types

### Construction

Syntax | Description
--- | ---
Example
    time_period(ptime,
                ptime)
| Create a period as [begin, end). If end is <= begin then the period will be defined as invalid.
    date d(2002,Jan,01);
    ptime t(d, seconds(10)); //10 sec after midnight
    time_period tp(t, hours(3));
    time_period(ptime,
                time_duration)
| Create a period as [begin, begin+len) where end would be begin+len. If len is <= zero then the period will be defined as invalid.
    date d(2002,Jan,01);
    ptime t1(d, seconds(10)); //10 sec after midnight
    ptime t2(d, hours(10)); //10 hours after midnight
    time_period tp(t1, t2);
    time_period(time_period rhs)
| Copy constructor
    time_period tp1(tp);

### Accessors

Syntax | Description
--- | ---
Example
    ptime begin()
| Return first time of period.
    date d(2002,Jan,01);
    ptime t1(d, seconds(10)); //10 sec after midnight
    ptime t2(d, hours(10)); //10 hours after midnight
    time_period tp(t1, t2);
    tp.begin(); // --> 2002-Jan-01 00:00:10
    ptime last()
| Return last time in the period
    date d(2002,Jan,01);
    ptime t1(d, seconds(10)); //10 sec after midnight
    ptime t2(d, hours(10)); //10 hours after midnight
    time_period tp(t1, t2);
    tp.last();// --> 2002-Jan-01 09:59:59.999999999
    ptime end()
| Return one past the last in period
    date d(2002,Jan,01);
    ptime t1(d, seconds(10)); //10 sec after midnight
    ptime t2(d, hours(10)); //10 hours after midnight
    time_period tp(t1, t2);
    tp.last(); // --> 2002-Jan-01 10:00:00
    time_duration length()
| Return the length of the time period.
    date d(2002,Jan,01);
    ptime t1(d); //midnight
    time_period tp(t1, hours(1));
    tp.length() --> 1 hour
    bool is_null()
| True if period is not well formed. eg: end is less than or equal to begin.
    date d(2002,Jan,01);
    ptime t1(d, hours(12)); // noon on Jan 1st
    ptime t2(d, hours(9)); // 9am on Jan 1st
    time_period tp(t1, t2);
    tp.is_null(); // true
    bool contains(ptime)
| True if ptime is within the period. Zero length periods cannot contain any points.
    date d(2002,Jan,01);
    ptime t1(d, seconds(10)); //10 sec after midnight
    ptime t2(d, hours(10)); //10 hours after midnight
    ptime t3(d, hours(2)); //2 hours after midnight
    time_period tp(t1, t2);
    tp.contains(t3); // true
    time_period tp2(t1, t1);
    tp2.contains(t1); // false
    bool contains(time_period)
| True if period is within the period
    time_period tp1(ptime(d,hours(1)),
                    ptime(d,hours(12)));
    time_period tp2(ptime(d,hours(2)),
                    ptime(d,hours(4)));
    tp1.contains(tp2); // --> true
    tp2.contains(tp1); // --> false
    bool intersects(time_period)
| True if periods overlap
    time_period tp1(ptime(d,hours(1)),
                    ptime(d,hours(12)));
    time_period tp2(ptime(d,hours(2)),
                    ptime(d,hours(4)));
    tp2.intersects(tp1); // --> true
    time_period intersection(time_period)
| Calculate the intersection of 2 periods. Null if no intersection.
    time_period merge(time_period)
| Returns union of two periods. Null if no intersection.
    time_period span(time_period)
| Combines two periods and any gap between them such that begin = min(p1.begin, p2.begin) and end = max(p1.end , p2.end).
    time_period shift(days)
| Add duration to both begin and end.

### Conversion To String

Syntax | Description
--- | ---
Example
    std::string
      to_simple_string(time_period dp)
| To `[YYYY-mmm-DD hh:mm:ss.fffffffff/YYYY-mmm-DD hh:mm:ss.fffffffff]` string where `mmm` is 3 char month name.
    [2002-Jan-01 01:25:10.000000001/
                    2002-Jan-31 01:25:10.123456789]
    // string occupies one line

### Operators

Syntax | Description
--- | ---
Example
    operator<<
| Output streaming operator for time duration. Uses facet to output [date time_of_day/date time_of_day]. The default is format is `[YYYY-mmm-DD hh:mm:ss.fffffffff/YYYY-mmm-DD hh:mm:ss.fffffffff]` string where `mmm` is 3 char month name and the fractional seconds are left out when zero.
    [2002-Jan-01 01:25:10.000000001/ \
        2002-Jan-31 01:25:10.123456789]
    operator>>
| Input streaming operator for time duration. Uses facet to read [date time_of_day/date time_of_day]. The default is format is `[YYYY-mmm-DD hh:mm:ss.fffffffff/YYYY-mmm-DD hh:mm:ss.fffffffff]` string where `mmm` is 3 char month name and the fractional seconds are left out when zero.
    [2002-Jan-01 01:25:10.000000001/ \
        2002-Jan-31 01:25:10.123456789]
    operator==, operator!=
| Equality operators. Periods are equal if p1.begin == p2.begin && p1.last == p2.last
    if (tp1 == tp2) {...
    operator<
| Ordering with no overlap. True if tp1.end() less than tp2.begin()
    if (tp1 < tp2) {...
    operator>
| Ordering with no overlap. True if tp1.begin() greater than tp2.end()
    if (tp1 > tp2) {... etc
    operator<=, operator>=
| Defined in terms of the other operators.

#### Time Iterators

Introduction -- Header -- Overview -- Operators

### Introduction

Time iterators provide a mechanism for iteration through times. Time iterators are similar to Bidirectional Iterators. However, time_iterators are different than standard iterators in that there is no underlying sequence, just a calculation function. In addition, time_iterators are directly comparable against instances of class ptime. Thus a second iterator for the end point of the iteration is not required, but rather a point in time can be used directly. For example, the following code iterates using a 15 minute iteration interval. The print hours example also illustrates the use of the time_iterator.

    	#include "boost/date_time/posix_time/posix_time.hpp"
    	#include <iostream>

    	int
    	main()
    	{
    	  using namespace boost::gregorian;
    	  using namespace boost::posix_time;
    	  date d(2000,Jan,20);
    	  ptime start(d);
    	  ptime end = start + hours(1);
    	  time_iterator titr(start,minutes(15)); //increment by 15 minutes
    	  //produces 00:00:00, 00:15:00, 00:30:00, 00:45:00
    	  while (titr < end) {
    	    std::cout << to_simple_string(*titr) << std::endl;
    	    ++titr;
    	  }
    	  std::cout << "Now backward" << std::endl;
    	  //produces 01:00:00, 00:45:00, 00:30:00, 00:15:00
    	  while (titr > start) {
    	    std::cout << to_simple_string(*titr) << std::endl;
    	    --titr;
    	  }
    	}

### Header

    #include "boost/date_time/posix_time/posix_time.hpp" //include all types plus i/o
    or
    #include "boost/date_time/posix_time/posix_time_types.hpp" //no i/o just types

### Overview

Class | Description
--- | ---
Construction Parameters
    time_iterator
| Iterate incrementing by the specified duration.
    ptime start_time, time_duration increment

### Operators

Syntax | Description
--- | ---
Example
    operator==(const ptime& rhs),
    operator!=(const ptime& rhs),
    operator>, operator<,
    operator>=, operator<=
| A full complement of comparison operators
    date d(2002,Jan,1);
    ptime start_time(d, hours(1));
    //increment by 10 minutes
    time_iterator titr(start_time, minutes(10));
    ptime end_time = start_time + hours(2);
    if (titr == end_time) // false
    if (titr != end_time) // true
    if (titr >= end_time) // false
    if (titr <= end_time) // true
    prefix increment
| Increment the iterator by the specified duration.
    //increment by 10 milli seconds
    time_iterator titr(start_time, milliseconds(10));
    ++titr; // == start_time + 10 milliseconds
    prefix decrement
| Decrement the iterator by the specified time duration.
    time_duration td(1,2,3);
    time_iterator titr(start_time, td);
    --titr; // == start_time - 01:02:03

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