C# - how to make an input mask for datagridview

As a "beginner" (or even if not), it's sometimes easier just to have separate columns for year, month and day when doing data entry (like: is it yyyymmdd; ddmmyyyy; yyyy-mm-dd; etc. etc.) The "user" won't mind if you don't make them use "more" keystrokes.

The data grid does not support the "masked edit control" (which is what you want).

And a "date" is not a date until it is "valid"; so forget about "formatting", and "validate" the "date columns" when ready to "save"; then combine to make a "date" for storing. "Split" it again, later, when editing again.