Well, I've asked them already. But they didn't know either.
I would honestly try as hard as you can to get as close as you can. If you want, put in an explanation saying that you aren't totally sure of the exact months/years but this was as close as you could get.
Reason: if they find out the dates were off, you could get an interview and possibly get the whole thing thrown out for misrepresentation. Might not happen, but not worth the risk!