How to calculate time duration in minutes using different formats

Hi,

Am new to this site which i find really awesome. Am not an expert in excel but i do like playing around with formulas. I have a quick (and hopefully easy) problem which I would want to ask help with:
I am calculating time duration (minutes) and both my Time Start (E7) and Time End (F7) are formatted as "Custom=> 0":"00" so it is easy to type the time (no need to manually press shift and colon). The simple formula for the duration (G7) is F7-E7 which results to 0:00 with the same format of custom=>0":"00. I have attached the sample file as basis to solicit some help?

Thanks!