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).

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, i.e. LW to mean "the last business day of the month."

#

# 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.

H

H is used to denote a hash, supported only when using WithHash, which takes in an additional Hash ID that will be hashed using a deterministic non-cryptographic hash function, such that the ID will always be hashed to give the same value in place of H.

If the cron expression contains a H symbol but no hash ID is provided, an error will be thrown during the ParseXX call.

This behaviour is inspired from Jenkins-specific cron syntax. Quoted directly from their documentation https://www.jenkins.io/doc/book/pipeline/syntax/#cron-syntax:

To allow periodically scheduled tasks to produce even load on the system, the symbol H (for “hash”) should be used wherever possible. For example, using 0 0 * * * for a dozen daily jobs will cause a large spike at midnight. In contrast, using H H * * * would still execute each job once a day, but not all at the same time, better using limited resources.

The H symbol can be used with a range. For example, H H(0-7) * * * means some time between 12:00 AM (midnight) to 7:59 AM. You can also use step intervals with H, with or without ranges.

The H symbol can be thought of as a random value over a range, but it actually is a hash of the job name, not a random function, so that the value remains stable for any given project.

Beware that for the day of month field, short cycles such as */3 or H/3 will not work consistently near the end of most months, due to variable month lengths. For example, */3 will run on the 1st, 4th, ... 31st days of a long month, then again the next day of the next month. Hashes are always chosen in the 1-28 range, so H/3 will produce a gap between runs of between 3 and 6 days at the end of a month. (Longer cycles will also have inconsistent lengths but the effect may be relatively less noticeable.)

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

Parse Options

Available options:

WithHash(hashID string)

Specifies the hash ID to hash, allows parsing of H.

If option not specified, it will fail when trying to parse any expression containing H.

WithHashEmptySeconds()

In the case when the seconds field is empty, it will interpret the missing field as H instead of 0.

Examples:

• Parse("H * * * *", WithHash(id)) will be parsed as 0 H * * * * *
• Parse("H * * * *", WithHash(id), WithHashEmptySeconds()) will be parsed as H H * * * * *
• Parse("0 * * * *", WithHash(id), WithHashEmptySeconds()) will be parsed as H 0 * * * * *

WithHashFields()

Using this parse option will introduce additional non-determinism into the hashed value. For example, given the following cron expression H H * * * * *, both the seconds and minutes place will always hash to the same value, such as 00:37:37, 01:37:37, etc.

Using WithHashFields() appends a suffix with the field descriptor's name to introduce an additional key to hash, such that any two field descriptors with the same interval size do not always hash to the same value.

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

License

License: pick the one which suits you best:

• GPL v3 see https://www.gnu.org/licenses/gpl.html
• APL v2 see http://www.apache.org/licenses/LICENSE-2.0