Maybe I'll get hammered for this reply, but I just think it doesn't really matter. You're supposed to complete the form to the best of your knowledge, which happens to be somewhat vague on this point, understandably. It would be a different situation if you couldn't quite remember whether you worked as a research assistant to a Nobel laureate one summer vs. flipped burgers, but I can't see that your end date of employment within a few months makes any difference. The application is meant to give people a general sense of what you have spent your time doing, and if there are any long stretches that are unaccounted for, those are probably worth explaining. But if you really just can't remember the exact month you left this one job, put down your best guess and then forget about it. I don't think anyone does more than skim over that section anyway.
Good luck.
