I would like to set up a template to include the following numbering:

1:00
1:01
1:02 etc up to 1:12
2:00
2:01
2:02
2:03 etc
3:00
3:01 etc up to 3:10
and so on.

I am using Word 2007. I can't find how to modify the existing styles or to create new styles to achieve the above.
I would appreciate any help.