Restrict absence to full or half day increments

Summary

Restricting absence to full or half day increments

Content

How can we restrict employees to entering only full day or half day absences based on their number of scheduled work hours?  We have employees on 8, 10 and 12 hour shifts.  Our work schedules that are both Time and Elapsed.  Does this need to be done in a fast formula? 

Thanks,

Ruth