What is a layman's alternative to the IT word "datetime"?

I would go with simply date and time. That is a phrase in common use, and would be commonly understood.

I think that in your situation, however, you might not need a variant of datetime; Rikon gives a couple example alternatives (Date Created or Date Modified) which describe the meaning of the date and time, rather than just saying date and time. This is more user-friendly in a spreadsheet.

I would just be more descriptive about what the date is... if it's a "time stamp" of some sort, then I think that's descriptive... I'm suspecting that it's a CreatedDate or ModifiedDate of some sort, in which case I would simlply say that: "Date Created" or "Date Modified"...

The fact that it includes the time doesn't need to be reflected in the column header...

EDIT

After evolving through this thought, many of us agree that the word date is not even necessary "Created", "Modified", "Ordered", etc.. is what I feel is the most concise way to do this. The fact that it's a datetime should be inferrable in most instances.

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