Restriction on backup periodicity

[florentk]

We would like define backup periodicity in minutes, hours, week, days, or months. If we use Cron, I think that add some restriction :

• The number of mimnute must be between 1 and 30,
• the number of hour between 1 and 12.
• the number of day between 1 and 14,
• the number of month between 1 and 6.

And we can't easly use the week interval.