Thank you Paresh Wagh! Thanks for the great help!

I had to convert the string type to a float. But when I reference the sum_50_overtime field with a related field, it doesn't get the value.

0 is the value of the sum_50_overtime field, but i don't know why.

But when I print to the console, it gets the correct value, only the related field does not.

sum_50_overtime = fields.Float(compute='_compute_sum_50')

@api.multi
@api.depends('overtime_50', 'sum_50_overtime')
def _compute_sum_50(self):
ots = []
for record in self:
ots.append(record.overtime_50)
tot = 0
for ot in ots:
t = parser.parse(ot)
tot += t.hour * 60 + t.minute
s = ('%s:%s' % (str(int(tot / 60)).zfill(2), str(tot % 60).zfill(2)))
time = float(s.replace(":", ""))
record.sum_50_overtime = time
print(record.sum_50_overtime)

The related field :

overtime_weekdays = fields.Float(related='attendance_ids.sum_50_overtime')

Thank you for your help!