Modifying an existing time formula

I have the following formula that I need adjusting and not sure what to do:

=IF(TIME(8,0,0)-A1=30/1440,TIME(8,0,0),A1)

If I enter 7:30 or 8:00 in cell A1 a value of 8:00 is returned. What I want to do now is also add 8:30. Which means when I enter 7:30, 8:00 or 8:30 the value of 8:00 should be returned.

Can one of you assist me with that?