Formatting of cells does not control the contents of cells, it just controls the fashion in which they are displayed. This is very important to note especially for dates. Formatting a cell as Date does not make the contents a date. The contents remain unchanged. Excel however does convert the contents to display in a format we are more accustom to seeing.
Excel stores dates as integers and times as decimal (or fraction of a day). If you enter 0.5 in a cell with general format, it will display as 0.5 However if you then change the format to Time is will display as 12:00 or what ever your local time format is set too. If you turn around and change the format back to general the display should be 0.5. Dates are stored as the number of days since 1900/1/1 with that date being 1. Again you can test this by entering 1 in a cell and then formatting it as date. If you are on a Mac I believe the year will be slightly different.
You state in your comment that your formula is:
=IF((AND(ISNUMBER(N108))), N108-G108, "")
This is great as you are testing the date in N108 to see if its a number or a string. If its a number you are performing your math operation on it and if it is not a number to return a blank. One thing you have not done is to test the value in G108 to see if it is a number or not as well.
On a side note, your AND in your formula is not currently serving any purpose as there is only 1 term inside the AND function. AND functions should be used when comparing 2 or more terms and either needing them to ALL be TRUE or At least one to be FALSE.
WITHOUT knowing what your sheet is doing I am guessing you want your formula to look something like:
=IF(AND(ISNUMBER(N108),ISNUMBER(G108)), N108-G108, "")
The above formula would test to see if both the value in N and G are numbers before trying to do the math operation. If either is not a number it will return blank.