Calculation of time periods
If you want to calculate the time between the beginning and the end of a temporary time period, the start point and the end point must have the same unit (years, days, hours, minutes or seconds etc.)
It is most convenient to use the smallest time unit being used in either the start time or the end time.
How many hours are there between 3 o’ clock pm and 6 o’ clock pm?
No smaller units than hours are mentioned in this question, and since both times are an indication of the hours that has passed since noon, it’s very easy to calculate, how much time the period last.
6 - 3 = 3
So 3 hours has passed in the time period 3-6 o’clock pm.
Here is a bit more complicated example:
How much time is there between 6:45 am and 9:35 am?
In this case we must convert the start time and the end time to total numbers of minutes that has passed since midnight, because it's the smallest unit that occurs in the two specifications of time.
There are 60 minutes in an hour, so the number of minutes from midnight to 6:45 is therefore:
and the number of minutes from midnight to 9:35 is:
Now it’s easy to calculate the difference in time, simply by subtracting start time form the end time:
So there is 170 minutes between 6:45 am and 9:35 am.