The Cron trigger lets you execute a macro periodically while you are logged in.
The Cron format lets you specify when a trigger will fire based on the time of day and and day of month and week.
The times are set using 5 (or 6) space separated fields, representing the
Each field can be a single number, or it can be a range (eg “3-7”) or a set of numbers or ranges separated by commas (eg “3,5-7,9”). Alternatively, the field can be “*” which matches all values. You can also add “/” and a number after a range to match every N'th value (eg “0-25/5” will match every five minutes in the first half of the each hour).
The trigger will match when all fields match the current time (as each minute ticks over). Exception: if either day of month or day of week is “” and the other isn't, then the trigger will match only the non- field, otherwise the trigger will match when either field matches.
For the Day of Week and Month fields, you can use the first three letters of the English name (eg “Jan” for January (1), or “Sun” for Sunday (0)).
If the number is negative, it matches from the end of the range (eg -1 in day of the month means the last day of the month).
If you start the day of the month field with a “+”, then the trigger will match only if both the day of the week and day of the month field matches. This allows you to match the last Friday of the month using “-1-7” for the day of the month, and “+Fri” for the day of the week.