|Don't be embarrassed. Dates and Times in Excel can be tricky.|
The background is this:
Excel stores Dates and Times as numbers, with the integer portion of the number being the day (using 1/1/1900 as Day 1) and the decimal portion being a fraction of a day.
For example, I just entered =NOW() in a cell and it displayed:
I formatted the cell as a number and it displayed:
because 10/25/2010 is the 40,476th day since the beginning of Excel time, and
.638603125 * 24 = 15.326475 or about 15 1/3 hours into the day.
That's why you can do "simple" math with Excel Dates and Times.
The point where it gets very tricky is when you are adding or subtracting times that cross the pesky Midnight threshold.