Data modelling for specific units of measurement
I would go with one unit (seconds).  Then I would have the input/output convert as needed.

Controlling cron from a table??  Turn it around.  Have the job continually running, but sleeping a computed length of time.  If necessary, use a table for communicating the "when to run next".  Be sure to watch out for edge cases -- such as being told that the next run time is yesterday.  It should probably be interpreted as means "run immediately".