It does seem pretty safe to assume that the ballot was signed sometime between the time it was printed and the date it was postmarked which, given the fast-paced and tumultuous nature of American politics, surely cannot be that wide of a window. And if the purpose is to somehow catch people submitting ballots after the cutoff date, surely one would expect a wouldbe election fraudster to lose no sleep in also falsifying an earlier date, making one wonder why this information is all that pertinent to begin with.
Now, a more cynical person might assume that this was just another one of those little traps specifically engineered to attract common mistakes (knowing full well that some of us are still putting the year down as "2015" because goddammit, what the hell time is it?) which can then be selectively enforced depending on whether or not you want to invalidate a large swath of votes. But I mean, surely our very trustworthy elected officials would never do such a thing...
In fairness that's probably exactly what that card would look like if either of those two had a hand in designing it.