Last entry date after automatic revalidation?

I am filling out immigration forms and I need to fill in the my last entry date to US. Well it is a little complicated issue. A few years ago I traveled to Canada for a short visit on an expired F-1 visa and came back using the automatic revalidation rule. When you do that, you do not surrender your I-94 so you do not receive a new one, and hence the travel is not recorded on your passport, though I know the actual date with 90% certainty. I checked old documents and saw that upon an immigration counselor's advice I did not mention that trip, that is I used the date from my previous entry, when I applied for OPT, and that was approved without a glitch. However there will be more scrutiny with a green card application and I really want to make sure that I put the correct date. Yet I called the USCIS with this question and instead of a meaningful answer I got a lesson on importance of being honest in my application. So do I use the actual last reentry date or the date on my passport?