code: def _convert_timestr_to_seconds(time_str, rec_start):
      if not CHECK_TIME_STR.match(time_str):
          raise ValueError('Input can only contain digits and colons')
      if ':' in time_str:
          time_split = [int(x) for x in time_str.split(':')]
          if len(time_split) == 2:
              time_split.append(0)
          clock_time = time(*time_split)
          chosen_start = datetime.combine(rec_start.date(), clock_time)
          if clock_time < rec_start.time():
              chosen_start += timedelta(days=1)
          window_start = int((chosen_start - rec_start).total_seconds())
      else:
          window_start = int(time_str)
      return window_start