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I am currently dealing with problems in which I need to find the difference between different times. One trick that worked for me until now was from here a short summary of which is

For Adding Times Add the hours Add the minutes If the minutes are 60 or more, subtract 60 from the minutes and add 1 to hours   For Subtracting Times Subtract the hours Subtract the minutes If the minutes are negative, add 60 to the minutes and subtract 1 from hours.  

However this trick logically fails when I attempt to subtract

5:00 PM - 12:00 PM (Ans 5 hours)

Also does anyone know how we can calculate differences like (2 AM - 5 PM (Day Before) ) Is the best way to calculate time difference to convert it in 24hr clock first ? I am actually in need of some speedy shortcuts for calculating these differences since I wont have a lot of time on my hands while solving these problems during a standardized test.

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    Working with a $24$-hour clock certainly makes it a lot easier.2012-06-29
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    When one crosses a division point (noon or midnight) I prefer to add. So from 5:00 PM yesterday to midnight, $7$. From midnight to 2:00 AM, $2$, total 9.2012-06-29

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