How do you pay an employee on a 16 or 24 hr schedule premium rates

We have firefighters who may work 56 hours in a week, therefore they may have multiple 16 hour days in a row. On days like holidays we are contractually obligated to pay them their 16 hours plus give them 16 hours comp time. To another group, we are obligated to pay them 16 hours plus pay them 16 hours premium pay for the day. How to do this has been a problem, since we can only put 24 hours on a day, therefore we have been having to split the hours and pay on days they are not actually working.