No date is stored the way it is displayed. Dates in FileMaker are actually integers that count the numer of days since 12/31/0000.
If you want to specify a date format for entering the date--as opposed to how it is displayed, you will need to get a bit creative.
You can set up a text field for data entry and a date field with an auto-enter calcualtion that converts the text entered in the text field into a true date.
If you have FileMaker 11, you can set up a script performed by the OnObjectValidate trigger to convert the YYMMDD text typed into the date field into a true date before the data validation kicks in and tries to tell you that you have entered an invalid date.
I knew the storage-stuff already, but I do find it weird that you cannot "sync" the display-format with the entry-format.
I - at least - would like those two synced for all my uses of dates. But thanks for the clarification on the non-existence of this ability. I'll be looking into your suggestion instead.