Subtracting Military Times

Date: 05/21/2000 at 07:05:40
From: Ben Connolly
Subject: Time calculation

Dear Dr. Math:

If you are given a start time and an end time of a phone call in 
minutes (e.g. start: 1756, end: 1812), how can you work out the 
duration in minutes?

Many thanks,
Ben Connolly

Date: 05/21/2000 at 12:37:47
From: Doctor Douglas
Subject: Re: Time calculation

Hi Ben,

Thanks for sending your question to Ask Dr. Math. In answering this 
question I am going to presume that the start and end times are given 
in military time format (e.g. 1756 means 5:56 P.M.).

Given the starting and ending time (1756, 1812), first separate them 
into hours and minutes:

     1756 = 17 hours and 56 minutes
     1812 = 18 hours and 12 minutes

Now subtract both parts (end - start):

       hours:  18-17 = 1 hour
     minutes:  12-56 = -44 minutes

Add these. In this case the number of minutes is negative, so we have 
to be careful and "borrow" one of the hours:

1 hour = 60 minutes, so upon adding we are left with +60-44 = 16 min., 
and the hours part is now zero, since we borrowed it to generate the 
60 minutes. The answer for the duration is 0 hours and 16 minutes.

Does that help? Please write back if you need more explanation.

- Doctor Douglas, The Math Forum
  http://mathforum.org/dr.math/