Calculate hours and minutes correctly

I've got a small issue when trying to calculate "worked hours"

In excel the data is formated as "time" and the cells have "6:00" "6:30" "8:30" but when I try to use formulas to calculate the total worked hours I keep getting 0 as results. If I change to , instead of : it works but there's constantly new hours added into the sheet and I wonder if there's some kind of fix for this issue?

Tldr: want to calculate hours and minutes, formated as time and with :

It works with 6,00 8,00 etc but not 6:00 8:00.