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: . Implementation -------------- The reference documentation for this implementation is found at , 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 `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 The `W` character is allowed for the day-of-month field. This character is used to specify the weekday (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 weekday 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. The `W` character can be specified only when the day-of-month is a single day, not a range or list of days. #### 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 , 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 Other details ------------- * If only six fields are present, a `0` second field is prepended, that is, `* * * * * 2013` internally become `0 * * * * * 2013`. * If only five fields are present, a `0` second field is prepended and a wildcard year field is appended, that is, `* * * * Mon` internally 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 Install ------- go get github.com/gorhill/cronexpr Usage ----- Import the library: import "github.com/gorhill/cronexpr" import "time" Simplest way: nextTime := cronexpr.Parse("0 0 29 2 *").Next(time.Now()) Assuming `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.Parse("0 0 29 2 *") nextTime := expr.Next(time.Now()) ... nextTime = expr.Next(nextTime) Use `time.IsZero()` to find out whether a valid time was returned. For example, cronexpr.Parse("* * * * * 1980").Next(time.Now()).IsZero() will return `true`, whereas cronexpr.Parse("* * * * * 2050").Next(time.Now()).IsZero() will return `false` (as of 2013-08-29...) You may also query for `n` next time stamps: cronexpr.Parse("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 API ---