Time calculations subtraction and average

ok folk I'm new here, so please bare with me. 1st problem concerns entering data as text and it being converted to time. I have found the formula 00\:00 which does an excellant job of converting. However, when I want to subtract, (=a2-a1) for example, it treats the time as a number. Sometimes it works, but if the hours are different, say 23:30- 22:10, it will subtract 2330-2210! Is there a way to make this work?
2nd problem, related to the first is when I try to avg a column of times, I get a similar effect. I believe the answer to the 1st may help resolve the 2nd, but I'm not sure. any have any thoughts?