valiadtion of date in dd.mm.yyyy format

i have a selection screen in which i want user to enter date in dd.mm.yyyy format only.how can i validate that.any FM.

i also want that date to be changed in yyyymmdd format coz i have to update that in a field which has yyyymmdd format.

so is there any fm for this also ( to change dd.mm.yyyy to yyyymmdd).

any pointers will help.