>But excel just incorrectly determines what you meant

How would you, if you were programming excel, determine whether the 5-digit number entered with a leading 0 is meant to be a zip code or not?

I would not make any default determination until and unless there was a proactive user action. If there is any value that doesn't round trip through string serialization, don't allow it to be coerced without the user deciding to allow it, explicitly.

You know that many users of Excel enter numbers into the cells and then do arithmetic calculations with them right?

No, but I think it's a lot. I'd bet that the vast majority of those round-trip through string serialization, requiring no further user action based on my recommendation.

Disagree, the most common way for an experienced user to type a date into Excel is as a partial date if it is this year, eg 4-Apr, which is instantly converted to 04/04/2025 or whatever.

The vast majority of dates I personally type into Excel would be in this form.

Format the column as date. Or turn on the global option for loose type insanity.

In this thread: people who don't know how to use Excel tell people who do know, how to use it.

Would keeping the leading zero cause any problems with future calculations?