[recursive]

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.

[dreghgh]

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

[recursive]

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.

[dreghgh]

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.

[recursive]

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

[dreghgh]

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