Welcome to Mylio! There are a few different fields for date/time in the XMP file:
If you edit the date of a photo in Mylio, all of these fields are updated in the XMP. If you set a date range (as opposed to a single date) only
MY:DateRangeEnd are updated.
If you only want the change to be reflected in Mylio, you can just edit those two fields (they can be set to the same value if you don’t want a range). If you want the change to be read by other programs, you should update the other fields as well.