Convert the two times to military time. 1 a.m. through 12 noon are written the same as civilian time. For 1 p.m. through 12 midnight, add 12 to the civilian time to convert to military time. If we want to subtract 11:30 a.m. from 3:45 p.m., 11:30 would stay the same. However, you need to add 12 to 3:45 so it would be 15:45.
Convert the two times to decimal format. The hours stay the same but the minutes need to be divided by 60 minutes. For 11:30, divide 30 by 60 to get .5. For 15:45, divide 45 by 60 to get .75.
Write the problem in normal subtraction format. Put the larger time on top and the smaller on the bottom. Align the decimal points and place a decimal for the answer in the same position. Add a 0 to .5 so the time has the same amount of digits to the right of the decimal. The answer will have two numbers after the decimal as well. Example: 15.75 - 11.50 = Answer
Subtract the bottom number from the top starting from the right and moving to the left. The answer to the example problem is 4.25 hours.
Convert the time back to its original format by multiplying .25 by 60 to get 15 minutes. Subtracting 11:30 a.m. from 3:45 p.m. is 4 hours and 15 minutes.