Using Sharepoint Online, I've created a new column on a document library "Expiry Date".
This worked fine until I changed the sites regional settings from English (United States) to English (United Kingdom) ( eg mm/dd/yyyy to dd/mm/yyyy).
Now when I try to enter a date into this column, I get error "Invalid datetime value". It even changes all currently entered dates to use the new format dd/mm/yyyy however if you try to change this you get the same error.
Changing the regional settings back makes the error go away but the column still in the wrong format mm/dd/yyyy.