Date and time

I am trying to do the following

1. Have Severity dropdown (1, 2, 3, 4 and 5)
2. Date (dateref)(varchar)
3. time (timeref) (varchar)

It must Automatically update date and time as per severity rating

a. Severity 1. Date to be same day, Time + 1hour
b. Severity 2. Date +1 day. Time to be current time
c. Severity 3. Date + 3 days. Time to be current time
d. Severity 4. Date + 5 days. Time to be current time
e Severity 5. Date and time to default blank.

If I select any severity, it must changed the time and date that is on the same form., and save it to the table.

Hope any one can help me