The following is an extract from linux contab man pages and gives you a explanation how to format the cron field. In our environment there is only a list of numbers or asterix allowed.

Commands are executed by cron(8) when the minute, hour,
and month of year fields match the  current  time,  and
when at least  one  of  the  two  day fields (day of
month, or day of week) match the current time examines
cron entries once every minute.  The time and date fields
are:

field          allowed values
-----          --------------
minute         0-59
hour           0-23
day of month   1-31
month          1-12 (or names, see below)
day of week    0-7 (0 or 7 is Sun, or use names)

A field may be an asterisk (*), which always stands for
first-last.

Here is a list of examples:

Run once a year, "0 0 1 1 *".
Run once a month, "0 0 1 * *".
Run once a week, "0 0 * * 0".
Run once a day, "0 0 * * *".
Run once an hour, "0 * * * *".
Run once an hour at five past, "5 * * * *".