I was just finishing up a remote session with a client today when she mentioned that she had one more thing to look at: the dates were wrong on most of her email in Mail.app.

I took a look and saw that all the email from today and yesterday showed the correct information. Today’s email showed the time it came in and all the email from yesterday showed “Yesterday”. Everything older than that showed 1/16/20. The mail was all there, it just had the wrong date.

Something seemed familiar with this, but I couldn’t place it. Usually, once I run into a problem and solve it, the answer is right there at my fingertips, but this was eluding me. I did a few searches and couldn’t find anyone with just this problem, but the feeling that I’d seen it before was growing. Something to do with time and date…

I went to System Preferences, then Language and Region. I noticed that her Region was set to “United States (custom). I clicked on the dropdown menu and changed it back to plain United States and we saw the dates of the emails correct right away.