Why monday.COM sum LAST LINE does not work with formula column

I set one of the columns as a function that calculates working hours for my employees in the office, giving it an entry time and an exit time . But in the “last line in SUM” she sums up the hours as zero and doesn’t sum them up .
Does the SUM line not work where the definition of the column is not numbers ?
لقطة شاشة 2023-12-16 113940

Yes they have to be numbers for the SUM to work on the group footer.

So T1 would need to be 0.5, 1.25, 1.25 respectively rather than 00:30, 01:15, 01:15.