Golang Cron expression parser
Given a cron expression and a time stamp, you can get the next time stamp which satisfies the cron expression.
In another project, I decided to use cron expression syntax to encode scheduling information. Thus this standalone library to parse and apply time stamps to cron expressions.
The time-matching algorithm in this implementation is efficient, it avoids as much as possible to guess the next matching time stamp, a common technique seen in a number of implementations out there.
There is also a companion command-line utility to evaluate cron time expressions: https://github.com/gorhill/cronexpr/tree/master/cronexpr (which of course uses this library).
The reference documentation for this implementation is found at https://en.wikipedia.org/wiki/Cron#CRON_expression, which I copy/pasted here (laziness!) with modifications where this implementation differs:
Field name Mandatory? Allowed values Allowed special characters ---------- ---------- -------------- -------------------------- Seconds No 0-59 * / , - Minutes Yes 0-59 * / , - Hours Yes 0-23 * / , - Day of month Yes 1-31 * / , - L W Month Yes 1-12 or JAN-DEC * / , - Day of week Yes 0-6 or SUN-SAT * / , - L # Year No 1970–2099 * / , -
Asterisk ( * )
The asterisk indicates that the cron expression matches for all values of the field. E.g., using an asterisk in the 4th field (month) indicates every month.
Slash ( / )
Slashes describe increments of ranges. For example
3-59/15 in the minute field indicate the third minute of the hour and every 15 minutes thereafter. The form
*/... is equivalent to the form "first-last/...", that is, an increment over the largest possible range of the field.
Comma ( , )
Commas are used to separate items of a list. For example, using
MON,WED,FRI in the 5th field (day of week) means Mondays, Wednesdays and Fridays.
Hyphen ( - )
Hyphens define ranges. For example, 2000-2010 indicates every year between 2000 and 2010 AD, inclusive.
L stands for "last". When used in the day-of-week field, it allows you to specify constructs such as "the last Friday" (
5L) of a given month. In the day-of-month field, it specifies the last day of the month.
W character is allowed for the day-of-month field. This character is used to specify the business day (Monday-Friday) nearest the given day. As an example, if you were to specify
15W as the value for the day-of-month field, the meaning is: "the nearest business day to the 15th of the month."
So, if the 15th is a Saturday, the trigger fires on Friday the 14th. If the 15th is a Sunday, the trigger fires on Monday the 16th. If the 15th is a Tuesday, then it fires on Tuesday the 15th. However if you specify
1W as the value for day-of-month, and the 1st is a Saturday, the trigger fires on Monday the 3rd, as it does not 'jump' over the boundary of a month's days.
W character can be specified only when the day-of-month is a single day, not a range or list of days.
W character can also be combined with
LW to mean "the last business day of the month."
Hash ( # )
# is allowed for the day-of-week field, and must be followed by a number between one and five. It allows you to specify constructs such as "the second Friday" of a given month.
Predefined cron expressions
(Copied from https://en.wikipedia.org/wiki/Cron#Predefined_scheduling_definitions, with text modified according to this implementation)
Entry Description Equivalent to @annually Run once a year at midnight in the morning of January 1 0 0 0 1 1 * * @yearly Run once a year at midnight in the morning of January 1 0 0 0 1 1 * * @monthly Run once a month at midnight in the morning of the first of the month 0 0 0 1 * * * @weekly Run once a week at midnight in the morning of Sunday 0 0 0 * * 0 * @daily Run once a day at midnight 0 0 0 * * * * @hourly Run once an hour at the beginning of the hour 0 0 * * * * * @reboot Not supported
- If only six fields are present, a
0second field is prepended, that is,
* * * * * 2013internally become
0 * * * * * 2013.
- If only five fields are present, a
0second field is prepended and a wildcard year field is appended, that is,
* * * * Moninternally become
0 * * * * Mon *.
- Domain for day-of-week field is [0-7] instead of [0-6], 7 being Sunday (like 0). This to comply with http://linux.die.net/man/5/crontab#.
- As of now, the behavior of the code is undetermined if a malformed cron expression is supplied
go get github.com/gorhill/cronexpr
Import the library:
import "github.com/gorhill/cronexpr" import "time"
nextTime := cronexpr.MustParse("0 0 29 2 *").Next(time.Now())
time.Now() is "2013-08-29 09:28:00", then
nextTime will be "2016-02-29 00:00:00".
You can keep the returned Expression pointer around if you want to reuse it:
expr := cronexpr.MustParse("0 0 29 2 *") nextTime := expr.Next(time.Now()) ... nextTime = expr.Next(nextTime)
time.IsZero() to find out whether a valid time was returned. For example,
cronexpr.MustParse("* * * * * 1980").Next(time.Now()).IsZero()
cronexpr.MustParse("* * * * * 2050").Next(time.Now()).IsZero()
false (as of 2013-08-29...)
You may also query for
n next time stamps:
cronexpr.MustParse("0 0 29 2 *").NextN(time.Now(), 5)
which returns a slice of time.Time objects, containing the following time stamps (as of 2013-08-30):
2016-02-29 00:00:00 2020-02-29 00:00:00 2024-02-29 00:00:00 2028-02-29 00:00:00 2032-02-29 00:00:00
The time zone of time values returned by
NextN is always the
time zone of the time value passed as argument, unless a zero time value is
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