get the number of hours or minutes between your two dates, then subtract the 6 hours for each day, and the number of days if you are closed on weekends.  Now you have the difference in say minutes from the start to the end, and then divide that number by the number of minutes between your open and close times.

 

This should at least get you close (you might want to account for work days or calendar days).

 

HTH

2) if the close time >1:30 am

  a) subtract (6*60)*datediff in days

 

should get you close.  Since it is 365/year most other issues drop away. Number of days might be tricky...if opened at 12:01 am and closed at 9:01 on the same day, the datediff in days =0, similiarly opened at 11:59pm and closed at 10:59 am the next day is 1 day diff, so it should work correctly, but you will have to check / code for the crossing of boundary times.

 

HTH