strtime: stable strptime / strftime for Go

strftime and strptime are functions found in nearly all C library implementations but with slightly different behaviors on each platform.

To provide identical behavior on all platforms where Go is supported, this strtime package provides two functions Strftime and Strptime which can be used in lieu of the platform's native C implementation.

To achieve this strtime:

• embeds and extends a Strftime function in Go, using an implementation originally from leekchan's timeutil package, which performs the format conversion natively in Go and is licensed under the terms of the standard MIT license; and
• embeds the standard FreeBSD implementation of strptime in C. FreeBSD's implementation is mature, robust, has the widest format specifier coverage, and its code is reusable under the terms of the permissive BSD license.

strtime is itself offered under the terms of the 2-clause BSD license.

How to use

func Strptime(value string, layout string) (time.Time, error)
func Strftime(t time.Time, layout string) (string, error)

Examples:

   start := "2016-10-20"
   t, _ := strtime.Strptime(start, "%Y-%m-%d")
   end, _ := strtime.Strftime(t, "%Y-%m-%d")
   Fmt.Println(start, end)

Supported format specifiers

Format	Description	Notes
%a	Short week day ("mon", "tue", etc)
%A	Long week day ("monday", "tuesday", etc)
%b	Short month name ("jan", "feb" etc)
%B	Long month name ("january", "february" etc)
%c	Equivalent to %a %b %e %H:%M:%S %Y
%C	Century	Only reliable for years -9999 to 9999
%d	Day of month 01-31
%D	Equivalent to %m/%d/%y
%e	Like %d but leading zeros are replaced by a space.
%f	Fractional part of a second with nanosecond precision, e.g. "123" is 123ms; "123456" is 123456µs, etc.	Strftime always formats using 6 digits.
%F	Equivalent to %Y-%m-%d
%h	Equivalent to %b
%H	Hours 00-23	See also %k
%I	Hours 01-12	See also %p, %l
%j	Day of year 000-366
%k	Hours 0-23 (padded with spaces)	See also %H
%l	Hours 1-12 (padded with spaces)	See also %I
%m	Month 01-12
%M	Minutes 00-59
%n	A newline character
%p	AM/PM	Only valid when placed after hour-related specifiers. See also %I, %l
%r	Equivalent to %I:%M:%S %p
%R	Equivalent to %H:%M	See also %T
%s	Number of seconds since 1970-01-01 00:00:00 +0000 (UTC)
%S	Seconds 00-59
%t	A tab character
%T	Equivalent to %H:%M:%S	See also %R
%u	The day of the week as a decimal, range 1 to 7, Monday being 1	See also %w
%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	See also %W
%w	The day of the week as a decimal, range 0 to 6, Sunday being 1	See also %u
%W	The week number of the current year as a decimal number, range 00 to 53, starting with the first Monday as the first day of week 01	See also %U
%x	Equivalent to %D
%X	Equivalent to %T
%y	Year without a century 00-99	Years 00-68 are 2000-2068
%Y	Year including the century
%z	Time zone offset +/-NNNN	Strftime always prints +0000
%Z	UTC or GMT	Strftime always prints UTC