Piang please help: Date of last Entry - no stamp on passport

I landed Canada in this May, returned to U.S. on 5/2/08 using H1B. There was no U.S. stamp on my passport showing the date I returned to U.S. Currently I am renewing my EAD. On line 12, Date of Last Entry into U.S., should I put down 5/2/08 or should I put down the date prior to that, which was in 1999?

Thanks for the help!