A CalendarSpec (almost) compliant calendar event library.
Because cron-spec is notoriously hard to read, not intuitive, and not standardized.
Note Shamelessly taken from System's documentation, with some changes to reflect what is or isn't impmemented as of the time of writing.
Calendar events may be used to refer to one or more points in time in a single expression.
Thu,Fri 2012-*-1,5 11:12:13
The above refers to 11:12:13 of the first or fifth day of any month of the year 2012, but only if that day is a Thursday or Friday.
The weekday specification is optional. If specified, it should consist of one or more English language weekday names, either in the abbreviated (Wed) or non-abbreviated (Wednesday) form (case does not matter), separated by commas. Specifying two weekdays separated by ".." refers to a range of continuous weekdays. "," and ".." may be combined freely.
Monday is considered the first day of the week and a range of weekdays must be
contained in the same week. For example, "every weekday except Thursday" cannot
be written Fri..Wed; it may be written Mon..Wed,Fri..Sun instead.
In the date and time specifications, any component may be specified as "*" in which case any value will match. Alternatively, each component can be specified as a list of values separated by commas. Values may be suffixed with "/" and a repetition value, which indicates that the value itself and the value plus all multiples of the repetition value are matched. Two values separated by ".." may be used to indicate a range of values; ranges may also be followed with "/" and a repetition value.
Either time or date specification may be omitted, in which case --* and 00:00:00 is implied, respectively. If the year component is not specified, "*-" is assumed. If the second component is not specified, ":00" is assumed.
A timezone specification may be added at the end of the expression. It can be
either UTC or a timezone as defined by the IANA (see
here for a
somewhat complete list).
Examples for valid timestamps and their normalized form:
Sat,Thu,Mon..Wed,Sat..Sun → Mon..Thu,Sat,Sun *-*-* 00:00:00
Mon,Sun 12-*-* 2,1:23 → Mon,Sun 2012-*-* 01,02:23:00
Wed *-1 → Wed *-*-01 00:00:00
Wed..Wed,Wed *-1 → Wed *-*-01 00:00:00
Wed, 17:48 → Wed *-*-* 17:48:00
Wed..Sat,Tue 12-10-15 1:2:3 → Tue..Sat 2012-10-15 01:02:03
*-*-7 0:0:0 → *-*-07 00:00:00
10-15 → *-10-15 00:00:00
monday *-12-* 17:00 → Mon *-12-* 17:00:00
Mon,Fri *-*-3,1,2 *:30:45 → Mon,Fri *-*-01,02,03 *:30:45
12,14,13,12:20,10,30 → *-*-* 12,13,14:10,20,30:00
12..14:10,20,30 → *-*-* 12..14:10,20,30:00
mon,fri *-1/2-1,3 *:30:45 → Mon,Fri *-01/2-01,03 *:30:45
03-05 08:05:40 → *-03-05 08:05:40
08:05:40 → *-*-* 08:05:40
05:40 → *-*-* 05:40:00
Sat,Sun 12-05 08:05:40 → Sat,Sun *-12-05 08:05:40
Sat,Sun 08:05:40 → Sat,Sun *-*-* 08:05:40
2003-03-05 05:40 → 2003-03-05 05:40:00
2003-02..04-05 → 2003-02..04-05 00:00:00
2003-03-05 05:40 UTC → 2003-03-05 05:40:00 UTC
2003-03-05 → 2003-03-05 00:00:00
03-05 → *-03-05 00:00:00
*:2/3 → *-*-* *:02/3:00
The main structure is Expression. It implements both encoding.TextMarhaler,
encoding.TextUnmarshaler, and fmt.Stringer, so it can be used in JSON
objects and stored in various ways.
There is also a func Parse(raw string) (exp Expression, err error) method to
parse a textual representation to an expression.