Everyday Math: How to know the time elapsed between StartTime and EndTime

Here is a simple calculation for everyday living.

Problem
How many hours are there from 9:30PM to 5:00AM, without running the hours in your fingers?
What about from 10:30AM to 4:00PM?

Solution
Case 1
If both StartTime and EndTime are in the AM or PM, simply subtract StartTime from EndTime

Example
Start Time: 8:00AM ; End Time 11:30AM 
Time Difference-> 11:30-8:00= 3 hours 30 mins

Case 2
If one is in the AM, the other in the PM
Formula: (12:00- Start Time) + End Time

Example 1
Start Time: 9:30PM ; End Time 5:00AM 
Time Difference: (12:00-9:30)+5:00 = 7hours 30mins

Example 2
Start Time: 10:30AM ; End Time 4:00PM 
Time Difference: (12:00-10:30)+4:00 = 5hours 30mins