In my spreadsheet, I have column A defined as format "time". There, I
insert values like:

0:00
13:20
7:30

etc.

In column B I have the formulae:

=A1+8.5
=A2+8.5

etc. (I want to add 8 1/2 hours to the start time.)

However, I'm not getting what I would expect. I would expect to see:

0:00 8:30
13:20 23:50
7:30 16:00

But instead I'm seeing:

0:00 12:00
13:20 1:20
7:30 19:30


What am I doing wrong?