date/time column format stuck - will not change if literally changed

Given they're left aligned, I’m going to guess (which is all I can do with a picture) that those cells are text, not a numeric Date/Time.

Try using Text to Columns to convert them.

Please try at J1

=--(MID(TEXT(I1,"mm/dd/"),4,3)&REPLACE(TEXT(I1,"mm/dd/yyyy hh:mm:ss"),3,3,))

You're welcome.

If you've got text in a cell, it doesn't matter what it looks like, or what Custom format you use, it isn't going to change.

If you'd posted a file, rather than a picture, I could have confirmed the diagnosis and tested a solution, or two. And others could have tested their solutions. It just makes life simpler and a solution more quickly available (and tested).


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Oh, and you're not changing the "data type", you're changing the way the underlying data is displayed. If it isn't numeric, you can't use a numeric format.