For events 24 hours or less in length, you can calculate the elapsed time using the European or military 24-hour clock by simply taking your start time and subtracting it from your end time. You can calculate the same way using the American 12-hour clock, but you will have to adjust your computation accordingly when time spans the midnight or noon hour.
The 24-hour clock used by the military and many European countries allows for straightforward calculations of elapsed time. For example, if your train from Paris leaves at 9:15 and arrives in Nice, France, at 13:45, you can determine the time elapsed by subtracting 9:15 from 13:45, resulting in four hours and 30 minutes.
If your train leaves Paris at 9:15 a.m. and arrives in Nice at 1:45 p.m. and you are using a 12-hour clock, you cannot simply subtract start time from end time, as this would result in an erroneously negative elapsed time. You can resolve this problem by converting the end time to its 24-hour-clock equivalent, or 13:45, or you can perform two measurements on either side of the 12 hour divide. For example, you could calculate the elapsed time from 9:15 a.m. to noon, for a result of two hours and 45 minutes, and the elapsed time from noon to 1:45, or one hour and 45 minutes, then add the two values together for a total of four hours and 30 minutes or four and one-half hours. You can double-check your calculation by plotting your times on a timeline.