Selection-Screen: Date Defaulted or User Specified?

On the selection-screen I want it to default to today's date - 90 days. So I have the fllowing code that works:

SELECTION-SCREEN BEGIN OF BLOCK process WITH FRAME TITLE text-001.

SELECTION-SCREEN SKIP 1.

PARAMETER: s_datum LIKE sy-datum OBLIGATORY.

SELECTION-SCREEN END OF BLOCK process.

AT SELECTION-SCREEN OUTPUT.

s_datum = sy-datum - 90.

The problem is I also want the user to be able to enter a date. But it keeps getting wiped out by my AT SELECTION-SCREEN OUTPUT event.

How do I work around this so it either defaults to sy-datum - 90 days, or the user can enter his own date?

Thank-You.