Hours & minutes in Excel

I need to record hours & minutes on a daily basis, and then calculate totals on a weekly/monthly basis total. Using the normal number formats doesn't work, because 60 minutes is shown as 0.60, not 1.00. So, 40 plus 40 is calculated as 80 not 1.20. Using the date/time formats doesnt work either, because the system wants to show the date. Similarly
Custom Format hh:mm doesn't work, because the resulting figures are shown as a date.
I know it's possible, because I've done it before. I just can't remember how
All help & ideas gratefully received.
Thanks
Jonathan Taylor