There is a quiet frustration baked into this question, and it is one we understand deeply. The user has built a complex tracker, done the hard work of structuring it, and then watched it all hinge on something as fallible as a human finger typing a date. The request is not for a magic wand. It is for a simple, visible signal: turn the cell red when the entry is wrong. That is not a small ask, and it is not a lazy one. It is the difference between a tool that works and a tool that nags.
The core of the problem is not the formula, though that is where the user is stuck. The core is that spreadsheets have long treated data validation as an afterthought, something you set up in a separate menu and hope people respect. The user tried ISDATE, hit a wall, and then tried to build a workaround with AND and ISNUMBER, only to get tangled in the logic. This is not a failure of effort. It is a failure of design. The formula bar should not be a place where good intentions go to die. It should be a place where you can say, plainly, "If this is not a proper date, make it red."
The good news is that the solution exists, and it is closer than it feels. The user is already on the right track with ISNUMBER, because dates in spreadsheet applications are stored as serial numbers. That means the real check is not whether the cell looks like a date, but whether it is a number within a reasonable range. A formula like =AND(ISNUMBER(A3), A3 < TODAY() + 3650) can catch dates that are missing a year or are wildly out of bounds. The key is to pair that with a clear conditional formatting rule, not a separate cell or a helper column. That way, the red highlight becomes a live warning, not a post-entry audit.
What this user is really asking for is accountability in the interface. They do not want to scold their users; they want the tool to do the scolding for them. That is a smart instinct, and it is one more people should follow. Stop building trackers that assume perfect input. Build trackers that assume imperfection and make the mistake visible on the spot. The formula will not stop every typo, but it will stop the quiet errors, the ones that sit in a cell and skew a whole column of totals. That is not just a nice-to-have. That is the difference between a spreadsheet that records data and one that protects it.