Time of loading elements

Hey

I have in one model two kind of slab - reinforced and post-tensioned so i need to assign them two different age of loading 

(28 days and 10 days) to get different creep coefficient. I made new material for PT, I mean the same parameters of concrete but new age of loading, but its changing globaly for the both o slabs. 

How to deal with it ? I will be gratefull for help.

Regards

Barbara