Hi,
I don't think it is possible to get the date field to do what you would like, the best solution I could think of would be to have 3 fields that are combined to make a single entry in a hidden field.
The user would see something like this
DropdownDayField (1-31) / DropDownMonthField(1-12) / 4 Digit Year field.
on change/exit of any of the fields you could run a script that checks which fields have been populated and create a date based on those values.
e.g. DropDownDayField = 3
DropDownMonthField = 5
4DigitYearField = 1967
hidden date Field == 03/05/1967
e.g. DropDownDayField = xx
DropDownMonthField = 5
4DigitYearField = 1967
hidden date Field == xx/05/1967
e.g. DropDownDayField = xx
DropDownMonthField = xx
4DigitYearField = 1967
hidden date Field == xx/xx/1967
This combined the Javascript Date object should be able to replicate what you would like.
Regards
Malcolm