Yes, 'T' in your timestamp format will be translated to either 'P' or 'A' as part of 'TT' resp. 'PM','AM'.
I think you can create your own format while keeping the numeric representation using dual() function:
[assuming a TimeStamp TS with a valid numeric representation]:
dual( Date(TS,'YYYY-MM-DD') &'T' & time(TS,'hh:mm:ss'), num(TS) ) as MyTimeStamp,