Crontab 时间计算

*  *  *  *  *
|  |  |  |  |
|  |  |  |  +----- day of week (0 - 7) (Sunday=0 or 7) OR sun,mon,tue,wed,thu,fri,sat
|  |  |  +-------- month (1 - 12) OR jan,feb,mar,apr ...
|  |  +----------- day of month (1 - 31)
|  +-------------- hour (0 - 23)
+----------------- minute (0 - 59)