Oh wow, great questions!

-Yes, there is a SERVICE_THRU_DATE. The time spans midnight when that field is not null.

-No need for different time zones.

-The start and end times can be at any time (not just on the 15 min mark).

-The staff can't bill for the same patient for 2 services at one time, however there are times one staff might work with a parent and one staff might work with a child, both billable for overlapping time for the same patient.