Thanks to Yvans post I have now a solution (his code did cause still an error):

@api.multi  # getting integer out of float from widget "float_time" 
def _float_time_convert(self, float_val):
  factor = float_val < 0 and -1 or 1
  val = abs(float_val)
  return (factor * int(math.floor(val)), int(round((val % 1) * 60)))

@api.one  # Getting the Stop DateTime
@api.onchange('start_datetime', 'toggle')
 def _get_stop_time_hrs(self):
    if not (self.start_datetime and self.duration) or self.allday is True:
    return value
    start_time = datetime.strptime(self.start_datetime, DEFAULT_SERVER_DATETIME_FORMAT)
    hour, minute = self._float_time_convert(self.duration)
    end_time = (start_time + timedelta(hours=hour,   minutes=minute)).strftime(DEFAULT_SERVER_DATETIME_FORMAT)
    self.stop_datetime = end_time

But I still get an strftime error when I try to get the result back to datetime field with return value from method (without onchange api ofcourse)?

 return end_time

Hi Lucas,

the problem is the type of self.start_datetime: it's a string and not as expected a datetime object. You have to convert the string to a datetime object before you add a timedelta. To to this, use the fields.Datetime.from_string method as shown below

end_time = fields.Datetime.from_string(self.start_datetime) + datetime.timedelta(hours=hours, minutes=minute)

Best regards

Yvan