Given a cron expression and a time stamp, you can get the next time stamp which satisfies the cron expression. It is based on the standalone Go library https://github.com/gorhill/cronexpr.

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         * / , -

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.

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.

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.

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.

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.

(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 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

go install github.com/fentas/cronexpr

Or download binary:

curl -sL -o /usr/local/bin/cronexpr "https://github.com/fentas/cronexpr/releases/download/0.1.0/cronexpr"
chmod +x /usr/local/bin/cronexpr

..as of 2015-11-07

Simplest way (returning unix timestamp ~ 1456700400)

cronexpr "0 0 29 2 *"

You may also query for n next time stamps

cronexpr -n 5 "0 0 29 2 *"

which returns the following time stamps

1456700400
1582930800
1709161200
1835391600
1961622000

The time zone of time values returned by default and --next, -n is always the time zone of your local time zone, unless --utc true is set. You can also format the output.

cronexpr -f "%c" -n 5 "0 0 29 2 *"

which returns the following

Mon, 29 Feb 2016 00:00:00 CET
Sat, 29 Feb 2020 00:00:00 CET
Thu, 29 Feb 2024 00:00:00 CET
Tue, 29 Feb 2028 00:00:00 CET
Sun, 29 Feb 2032 00:00:00 CET

At last you are able to set the time

cronexpr -f "%c" -u 1456700400 "0 0 29 2 *"

Resulting in

Sat, 29 Feb 2020 00:00:00 CET

Ideas and pull requests are always welcome.

• AGPL v3 see https://www.gnu.org/licenses/agpl-3.0.de.html