add 9 hours to actual value

Hi,

We have a drop down list that contains time value every half hour; so there are 48 total items.
items are :
00:00:00
00:30:00
01:00:00
01:30:00
02:00:00
02:30:00
03:00:00
03:30:00
04:00:00
04:30:00
05:00:00
05:30:00
06:00:00
06:30:00
07:00:00
07:30:00
08:00:00
08:30:00
09:00:00
09:30:00
10:00:00
10:30:00
11:00:00
11:30:00
12:00:00
12:30:00
13:00:00
13:30:00
14:00:00
14:30:00
15:00:00
15:30:00
16:00:00
16:30:00
17:00:00
17:30:00
18:00:00
18:30:00
19:00:00
19:30:00
20:00:00
20:30:00
21:00:00
21:30:00
22:00:00
22:30:00
23:00:00
23:30:00

How can we add 9 hours to the selected item in the drop down list and place it on a textbox1.text ?
i.e.
23:30 + 9 hours = 08:30

set auto post back on DDL and textbox1 ?
and on click DDL event place some code..

Please kindly guide me in what code to place in asp.net vb

Thanks.