I have a field in a form that subtracts one time from another in the form but it does not show up in the table. i am wondering if i have not created the table correctly

Does it need to appear in the table? You could simply calculate the value every time you needed it.

I have had a similar problem and have used Update Queries to overcome it. However these can be slow if you have loads of data and updates the whole column every time.

It is not usually a good idea (good db design) to store calculated values.