Good question.
TH = Throalic calendar (measured from the founding of Throal; this is the "default" calendar used for Earthdawn)
TE = Theran calendar (measured from the founding of Thera)
You can find this information in the ED3 Gamemaster's Guide, On the Passing of Days and Months, p.27.
As for what they stand for exactly, that's not explained.

TH = THroalic reckoning? TE = Theran Era? BT stands for Before Throal.
Take kaer, James