I need a column that contains the "Warning Date", which is two weeks prior to the expiry date. I am new to calculated columns so am not quite sure how to go about this. Any help is greatly appreciated.
List and description of date functions you can use in calculated column formula can be found on MSDN:
In your case, the formula will obviously look like this:
Column1 should be replaced with the name of your expiry date column.
Also please don't forget to set the calculated field output type to "Date and time".