F1 to H1B to EB-13

Situation: A foreign national works for a multinational company (MNC) abroad, then studies in the US (F1 visa), re-joins the MNC in the US on F1-OPT and later switches to H1B.

Question: The MNC wants to sponsor a Greencard. When does the 3-year period end during which the foreign national must have spent 12 months working abroad for the MNC to satisfy the EB-13 criteria? Would it be the b[/b] F1 petition date, (b) H1B petition date, or b [/b]the actual Greencard petition?