Voir ici :
http://www.nncron.ru/help/EN/working/cron-format.htm

Quelques exemples intéressants :
45 17 7 6 * 2001,2002 Once a year, on June 7th at 17:45, if the year is 2001 or 2002
0,15,30,45 0,6,12,18 1,15,31 * 1-5 * At 00:00, 00:15, 00:30, 00:45, 06:00, 06:15, 06:30,
06:45, 12:00, 12:15, 12:30, 12:45, 18:00, 18:15,
18:30, 18:45, on 1st, 15th or 31st of each month, but not on weekends
*/15 */6 1,15,31 * 1-5 * Same as above
0 9 1-7 * 1 * First Monday of each month, at 9 a.m.
0 0 1 * * * At midnight, on the first day of each month