Function Modules for time subtraction.

Question

Hello abapers..
I want two function modules...
1)  to subtract 1 hour from time and date.
for eg if the date entered is 23.3.08 and time is 00:00:00
then the function module should give the result as
22.3.08 and 23:00:00.
or if the date is 21.03.08 and time is 12:00:00.
The result should be 21.03.08 and time 11:00:00.
2) the second one is for validation..i am not sure if there is
a function module for that.
I want to validate whether the date entered by the user
in the select option for type sy-datum is in the correct format.
I mean the user should not enter anything except a date.
For eg:
If the user enters  97898jjjj then it should also invalidate this
as well as if the user enters 34.07.08 then also the FM should invalidate this..
Points will be rewarded for helpful answer.

Answer 1

Try this
DATA : diff TYPE i.
PARAMETERS :
       iv_stime TYPE  t,
       iv_sdate TYPE  d,
       iv_atime TYPE  t.
ev_endtime = iv_stime - iv_atime.
diff = ev_endtime - iv_stime.
IF diff > 0.
   ev_enddate = iv_sdate - 1.
ELSE.
   ev_enddate = iv_sdate.
ENDIF.
WRITE : ev_endtime.
WRITE : ev_enddate.