.xls or .xlsx in excel stores date values as days elapsed since the beginning of the epoch. So when did the epoch start? well excell has two answers for you, 1900 and 1904 depending on your configuration. The history is a bit long but you can dive into specifics here.

Why Excel Epoch Matter?

If you're trying to read a modern xls file on an older site, this should cause all of your dates to be off by 4 years and one day (leap year and all that jazz). But sometimes it will be just one day.

You can actually check the format of your dates here:

How To Fix It?

The best way to fix it, unless you want to update the reader, is to just deal with the off by one error directly. Some readers can actually alter between their reading mod so it's worth checking that out. But honestly once you know what is happening, and now i hope you do, fixing it is just a matter of preference.