How to Manually Calculate a Time Interval
Manual calculations are time-consuming and prone to errors, so we recommend avoiding them when possible. Still, there may be occasions where you need to manually calculate a time duration interval. Here’s how;
Step 1: Determine the start and end times
Let’s say your start time is 9:30 AM and your end time is 2:15 PM.
Step 2: Convert the time to a 24-hour (a.k.a. military time) format
To do this, add 12 to the hours of any PM times, or use our military time converter. In this case, 9:30 AM would become 0930, and 2:15 PM would become 1415.
Step 3: Calculate the time difference
Subtract the start hours from the end hours to calculate the time difference (e.g., 1400 - 900 = 500). Then, do the same with minutes (e.g., 15 - 30 = -15).
Step 4: Handle any borrow or carry-over
If the minute value of the ending time is smaller than the minute value of the starting time, you'll need to "borrow" an hour from the hour value of the ending time. If the ending hour is a smaller value than the starting hour, the value of the difference will be a negative number. In this case, you should add 24 hours to the result.
So, in our example, since subtracting minutes from the ending time yielded a negative number (-15), we should add 60 to the minutes, then adjust the hours. So, instead of -15, we’ll have 45, and instead of 500, we’ll have 400.
First, add the minutes to the hours. From there, you can convert the format from military time to standard hours and minutes or to decimal format. In our example, the result is 445, which is 4 hours and 45 minutes, or 4.75 hours.
