That printed date is the one that matters, not a general expectation carried over from a previous year, since the mailing date and the exact deadline can shift from one assessment cycle to the next. Reading the notice itself, rather than assuming, avoids missing the window entirely.
Missing it usually means waiting for the next assessment notice rather than getting an exception, since the Board of Revision works from the filing deadline on record, not from when an owner happens to notice the bill feels high.
If you have not checked your current notice for its own deadline, that is the first thing worth doing before anything else. Get your hand-reviewed valuation from Joel Dyck.