Cron

Basics

Each cron trigger is specified with a combination of six (or 7) white-space separated fields that dictate when the event should occur. In order, the fields specify trigger times for the second, minute, hour, day of the month, month, day of the week, and the year (optional)

.------------------ Second (0 - 59)
|   .--------------- Minute (0 - 59)
|   |   .------------ Hour (0 - 23)
|   |   |   .--------- Day of the month (1 - 31)
|   |   |   |   .------ Month (1 - 12) or Jan, Feb ... Dec
|   |   |   |   |   .---- Day of the week (0 (Sun.; also 7) - 6 (Sat.))
|   |   |   |   |   |   .- Year (optional) 
V   V   V   V   V   V   V
*   *   *   *   *   *   *

If the hour, minute, and month of a given time period are valid values as specified in the trigger and either the day of the month or the day of the week is a valid value, the trigger fires.

Ranges and Wild-cards

Ranges specify a starting and ending time period. It includes all values from the starting value to and including the ending value.

Wild-cards ("*") in a field represents all valid values.

The following cron expression is triggered every day at noon from June through September:

0 12 * 6-9 * *

If the day of the week field is a wild card, but the day of the month is an explicit range, the day of the week will be ignored and the trigger will only be activated on the specified days of the month. If the day of the month is a wild card, the same principal applies.

This expression is triggered every week day at 4:00 PM: 0 16 * * 1-5 *

This one is triggered the first nine days of the month: 0 16 1-9 * * *

This one is triggered every day for the first week, but only on Saturdays thereafter: 0 16 1-7 * 6 *

Steps

Steps are specified with a "/" and number following a range or wild-card. When iterating through a range with a step, the specified number of values will be skipped each time. 1-10/2 is the functional equivalent to 1,3,5,7,9.

The following cron expression is triggered on the first day of every quarter (Jan., Apr., ... Oct.) at midnight:

0 0 1 */2 * *

Monotonic Triggers

In typical cron implementations, setting the hour field to "*/9" would mean cause it to match the hours 00:XX, 09:XX and 18:XX. The following day, pattern would begin again starting from 00:XX making it impossible to easily define an event that occurs every 9 hours. Monotonic triggers are a solution to this problem; "%" in any field except the day of the week can be used to denote expressions that should happen every N-intervals.

Monotonic expressions in the year, month and day field use calendar values. For example, using "%15" in the day field with an epoch set to January 1st, 2017 would cause the days to match on January 1st, 16th and the 31st regardless of the time zone.

As per default, monotonic expressions match always with 0, one can also specify an offset. For example, the expression

%7 * * ? * *

Triggers every 7 seconds, starting at 0 seconds, so triggers at :0, :7, :14, etc. Whereas the monotonic trigger with offset 7

7%7 * * ? * *

triggers at :7, :14, :21, etc :14, :21, etc