I have a document in which I am trying to determine the length of time between a start date & time and a finish date & time. The format of the time/date cells is

mm/dd/yy hh:mm:ss

Basically, I am just subtracting the first cell from the second. This works fine as long as both dates are in the same day, such as

START FINISH ELAPSED
08/20/14 23:42:22 08/20/14 23:43:59 0:01:37

However, if I have a situation such as

START FINISH
08/18/14 23:00:15 08/19/14 0:03:22

the ELAPSED cell fills with ######## with a tooltip stating that "dates and times that are negative or too large display as ######." The actual result in this example should be 1:03:07.

How do I get consistency amongst my resulting formulas?