By putting the parentheses around the time figure, you will probably be
converting it into a text value, so it will not be considered in the
normal summing. I think the way around this would be to use a helper
column to indicate if the time is to be subtracted or added (you can
use conditional format to turn the cell red if the cell next to it
contains "-" or whatever symbol you choose to indicate subtraction. I
put this data in cells A1 to B6:

0:08:00
0:06:00
0:03:00
0:09:00 -
0:05:00
0:07:30 -

Then in another cell I put this array formula*:

=SUM(IF(B1:B6="-",-A1:A6,A1:A6))

* As this is an array formula, when you have typed it in (or when you
subsequently edit it) you must use CTRL-SHIFT-ENTER instead of ENTER.
If you do this correctly then Excel will wrap curly braces { } around
the formula - you must not type these yourself.

The value I got was 0:05:30, which is correct for this data.

Hope this helps.

Pete