counting based on criteria

[SOT]

I need to count the cells with a : in them, but only from the first entered
time to the last entered time.

Please see example:

MON TUE WED THU FRI
09:00 12:00 09:00 : 09:30 result should be 1
: 13:55 : 10:20 13:20 result should be 1
10:55 10:30 : 10:55 : result should be 1
: 13:00 : : 13:50 result should be 2
09:50 : 10:00 : : result should be 1

can anyone suggest a function or functions I could try?

Many thanks.