boost日期格式互轉和time_t互轉

boost日期庫雖然強大，使用起來不太方便，在此記錄下日期轉換代碼。

boost日期格式轉換代碼如下：

 bool FromString(boost::posix_time::ptime& pt, std::string datetime, std::string format)
{
    std::stringstream ss(datetime);
    //std::locale responsible for releasing memory.
    ss.imbue(std::locale(ss.getloc(), new boost::posix_time::time_input_facet(format)));//out
    if ( ss >> pt )
    {
        return true;
    }
    return false;
}


 bool ToString(const boost::posix_time::ptime& pt, std::string& datetime, std::string format)
{
    std::stringstream ss;
    //std::locale responsible for releasing memory.
    ss.imbue(std::locale(ss.getloc(), new boost::posix_time::time_facet(format)));//out
    if ( ss << pt )
    {
        datetime = ss.str();
        return true;
    }
    return false;
}

std::time_t ToUtcTime(const boost::posix_time::ptime& pt)
{
    boost::posix_time::time_duration offset(
        boost::posix_time::second_clock::local_time() - boost::posix_time::second_clock::universal_time());
    boost::posix_time::ptime epoch(boost::gregorian::date(1970, 1, 1));
    boost::posix_time::ptime putc = pt - offset;
    boost::posix_time::time_duration diff(putc - epoch);
    return diff.total_seconds();
}

//FromUtcTime => boost::posix_time::from_time_t(std::time_t time);
//FromTM => ptime ptime_from_tm(tm timetm)
//ToTM => tm to_tm(time_duration)

日期格式文檔如下：

Date Facet Format Flags

Format Specifier	Description
	Example
%a	Abbreviated weekday name
	"Mon" => Monday
%A	Long weekday name
	"Monday"
%b	Abbreviated month name
	"Feb" => February
%B	Full month name
	"February"
%c !	The preferred date and time representation for the current locale.
%C !#	The century number (year/100) as a 2-digit integer.
%d	Day of the month as decimal 01 to 31. When used to parse input, the leading zero is optional.
%D !#	Equivalent to %m/%d/%y
%e #	Like %d, the day of the month as a decimal number, but a leading zero is replaced by a space. When used to parse input, the leading space is optional.
%G !	This has the same format and value as %y, except that if the ISO week number belongs to the previous or next year, that year is used instead.
%g !	Like %G, but without century.
%h !#	Equivalent to %b
%j	Day of year as decimal from 001 to 366 for leap years, 001 - 365 for non-leap years.
	"060" => Feb-29
%m	Month name as a decimal 01 to 12
	"01" => January
%u !	The day of the week as a decimal, range 1 to 7, Monday being 1.
%U	The week number of the current year as a decimal number, range 00 to 53, starting with the first Sunday as the first day of week 01. In 2005, Jan 1st falls on a Saturday, so therefore it falls within week 00 of 2005 (week 00 spans 2004-Dec-26 to 2005-Jan-01. This also happens to be week 53 of 2004).
	date d(2005, Jan, 1); // Saturday // with format %U ss << d; // "00" d += day(1); // Sunday ss << d; // "01" beginning of week 1
%V !#	The ISO 8601:1988 week number of the current year as a decimal number, range 01 to 53, where week 1 is the first week that has at least 4 days in the current year, and with Monday as the first day of the week.
%w	Weekday as decimal number 0 to 6
	"0" => Sunday
%W	Week number 00 to 53 where Monday is first day of week 1
	date d(2005, Jan, 2); // Sunday // with format %W ss << d; // "00" d += day(1); // Monday ss << d; // "01" beginning of week 1
%x	Implementation defined date format from the locale.
	date d(2005,Oct,31); date_facet* f = new date_facet("%x"); locale loc = locale(locale("en_US"), f); cout.imbue(loc); cout << d; // "10/31/2005" loc = locale(locale("de_DE"), f); cout.imbue(loc); cout << d; // "31.10.2005"
%y	Two digit year
	"05" => 2005
%Y	Four digit year
	"2005"
%Y-%b-%d	Default date format
	"2005-Apr-01"
%Y%m%d	ISO format
	"20050401"
%Y-%m-%d	ISO extended format
	"2005-04-01"

Time Facet Format Flags

Format Specifier	Description
	Example
%- *!	Placeholder for the sign of a duration. Only displays when the duration is negative.
	"-13:15:16"
%+ *!	Placeholder for the sign of a duration. Always displays for both positive and negative.
	"+13:15:16"
%f	Fractional seconds are always used, even when their value is zero
	"13:15:16.000000"
%F *	Fractional seconds are used only when their value is not zero.
	"13:15:16" "05:04:03.001234"
%H	The hour as a decimal number using a 24-hour clock (range 00 to 23).
%I !	The hour as a decimal number using a 12-hour clock (range 01 to 12).
%k !	The hour (24-hour clock) as a decimal number (range 0 to 23); single digits are preceded by a blank.
%l !	The hour (12-hour clock) as a decimal number (range 1 to 12); single digits are preceded by a blank.
%M	The minute as a decimal number (range 00 to 59).
%O	The number of hours in a time duration as a decimal number (range 0 to max. representable duration); single digits are preceded by a zero.
%p !	Either `AM' or `PM' according to the given time value, or the corresponding strings for the current locale.
%P !#	Like %p but in lowercase: `am' or `pm' or a corresponding string for the current locale.
%r !#	The time in a.m. or p.m. notation. In the POSIX locale this is equivalent to `%I:%M:%S %p'
%R !	The time in 24-hour notation (%H:%M)
%s *	Seconds with fractional seconds.
	"59.000000"
%S	Seconds only
	"59"
%T !	The time in 24-hour notation (%H:%M:%S)
%q	ISO time zone (output only). This flag is ignored when using the time_facet with a ptime.
	"-0700" // Mountain Standard Time
%Q	ISO extended time zone (output only). This flag is ignored when using the time_facet with a ptime.
	"-05:00" // Eastern Standard Time
%z *!	Abbreviated time zone (output only). This flag is ignored when using the time_facet with a ptime.
	"MST" // Mountain Standard Time
%Z *!	Full time zone name (output only). This flag is ignored when using the time_facet with a ptime.
	"EDT" // Eastern Daylight Time
%ZP *	Posix time zone string (available to both input and output). This flag is ignored when using the time_facet with a ptime. For complete details on posix time zone strings, see posix_time_zone class.
	"EST-05EDT+01,M4.1.0/02:00,M10.5.0/02:00"
%x %X	Implementation defined date/time format from the locale.
	date d(2005,Oct,31); ptime pt(d, hours(20)); time_facet* f = new time_facet("%x %X"); locale loc = locale(locale("en_US"), f); cout.imbue(loc); cout << pt; // "10/31/2005 08:00:00 PM" loc = locale(locale("de_DE"), f); cout.imbue(loc); cout << pt; // "31.10.2005 20:00:00"
%Y%m%dT%H%M%S%F%q	ISO format
	"20051015T131211-0700" // Oct 15, 2005 13:12:11 MST
%Y-%m-%d %H:%M:%S%F%Q	Extended ISO format
	"2005-10-15 13:12:11-07:00"
%Y-%b-%d %H:%M:%S%F %z	Default format used when outputting ptime and local_date_time.
	"2005-Oct-15 13:12:11 MST"
%Y-%b-%d %H:%M:%S%F %ZP	Default format used when inputting ptime and local_date_time.
	"2005-Oct-15 13:12:11 MST-07"
%-%H:%M:%S%F !	Default time_duration format for output. Sign will only be displayed for negative durations.
	"-13:14:15.003400"
%H:%M:%S%F	Default time_duration format for input.
	"13:14:15.003400"