SQL insert statement help please

Hi all I'm trying to go a little out of my comfort level here and was hoping for some advice. I have a sql table that has 2 date fields. an add and a modified. If the modified date is null then I want to calculate on the date added. Otherwise I'd like to calculate on the modified date.

Here is a simplified version of what I'm trying to accomplish.

Hope it makes sense what I'm trying to do.

SELECTfield1,field2....,dateAdded,dateModifiedINTOtblOutputFROMtblInput/* some pseudo code here */WHERE(ifdateModifiednotnullthen(dateModified<={fnNOW()}-60)else(dateAdded<={fnNOW()}-60)