cronexpression for Go

Cron expression parser in Go language (golang).

Given a cron expression and a time stamp, you can get the next time stamp which satisfy the cron expression.

In another project, I decided to use Cron syntax to encode scheduling information. Thus this standalone library to parse and execute cron expressions.

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 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 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 five fields are present, a wildcard year field is appended, that is, * * * * * internally become 0 * * * * * *.
• If only six field are present, 0 is prepended as second field, that is, * * * * * 2013 internally become 0 * * * * * 2013.
• 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/cronexpression

Usage

Import the library:

import "github.com/gorhill/cronexpression"
import "time"

Simplest way:

nextTime := cronexpression.NextTimeFromCronString("0 0 29 2 *", time.Now())

Assuming time.Now() is "2013-08-29 09:28:00", then nextTime will be "2016-02-29 00:00:00".

If you need to reuse many times a cron expression in your code, it is more efficient to create a CronExpression object once and keep a copy of it for reuse:

cronexpr := cronexpression.NewCronExpression("0 0 29 2 *")
nextTime := cronexpr.NextTime(time.Now())