It's difficult to answer your query, simply because it's ambiguous in nature! in which context would you want to know about TAT? You haven't provided any details which could help in answering it!!

TAT could also mean Thematic Apperception Test which a projective kind psychological test. 

If you give out other details, I'm sure you will have many members willing to help you.TAT is nothing but Turn Around Time, i.e, time taken to complete given task or how quick you work on given requirement