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/cronexprdo (which of course uses this library).

Implementation

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

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 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.

The W character can be specified only when the day-of-month is a single day, not a range or list of days.

The W character can also be combined with L 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

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.MustParse("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.MustParse("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.MustParse("* * * * * 1980").Next(time.Now()).IsZero()

will return true, whereas

cronexpr.MustParse("* * * * * 2050").Next(time.Now()).IsZero()

will return 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 Next and NextN is always the time zone of the time value passed as argument, unless a zero time value is returned.

API

http://godoc.org/github.com/gorhill/cronexpr