There are 2 extensions which provide a different calendar with a different date format. However the number of characters in the date format is too limited to allow them defining them.
So one of them uses a _ (underscore) to prefix the day/month/year symbols of their own format which brings them to the 30 character limit of the database field.
Previously the mods just increased the field, however since the html5 patch the input fields also have the max length set and this can not be modified.
So I suggest, to simply increase the field size directly to maybe 48 or 64 characters.