Sliding date support

Add cron-driver sliding date support.

The feature would allow selecting between fixed date or sliding date. When the sliding date option selected, the date-time selector changes to relative date-time period selector. Ones saved, on the next cron run the date would change accordingly to the settings. Granularity option should allow to control the frequency of change.

E.g. the sliding date option is set to -7 days. Granularity set to every 1 hour. Cron can be set to run as frequently as desired. On each cron run, the cut-off date would be adjusted relative to the current time, the frequency of adjustments would depend on the granularity settings, in the given example the adjustment would happen every hour.