Date Format not working with equations

Hi,

I am looking at a side project and have been trialling some potential spread betting strategies within a FX trading company. This company allows you to export a lot of the data into excel but doesn't analyze it in every way that I want.

One of the biggest things with the data would be the ability to manipulate the dates to give information such as the average length of trade being open and also having the ability to filter out to isolate trading weeks / days etc.

For some reason the date within the excel sheet doesn't seem to be working and I cannot get it to do simple functions such as Closing Trade time - Opening trade time to give a length of time the trade is open.

Any help on this would be appreciated, and where possible a quick explination of how you solved it as I want to run the report on a weekly basis and don't want to have to be coming back here bugging everyone to get it updated!

Many thanks
Maudise