|You could begin with this page, it will explain many of the Date Math functions you could use.|
We would need more information on how your worksheet is set up,
such as what data is in what cells.
Usually a short example of your sheet with data helps.
To post an short example, use the Pre tags that show at the top of the reply window. Simply press the pre tag icon and place your example between the tags.
You can also preview your post before you actually post it by pressing the Preview Follow Up button next to the Submit button.