A quick and dirty way for doing this. No error checking, big assumptions about the input to the prompt being right.
nd := @Prompt([OKCANCELEDIT]; "Enter the New Date"; "YYYY/MM/DD."; @Today);
@If (nd ="";@Return("");"");
y := @Left (nd;"/");
m := @Left(@Right (nd;"/");"/");
d:= @right(@Right (nd;"/");"/");
FIELD DateFieldOnForm := @Date(@TextToNumber (y) ;@TextToNumber ( m)
;@TextToNumber ( d) )
the Key is parsing the string with @lefts and such, and then TextTonumber, to put humpty Dumpty back together again. The @Date means it is a Date field on the document, not a text string.
The better, clean, way would be to use LotusScript and open a form in Dialog box mode, the form having a Calander tool on it.