code: def validate_time_range(form, field):
      
  
      string = field.data
      try:
          begin, end = string.split('-')
          begin = begin.strip()
          end = end.strip()
          begin_hours, begin_min = begin.split(':')
          end_hours, end_min = end.split(':')
          assert 0 <= int(begin_hours) <= 23
          assert 0 <= int(end_hours) <= 23
          assert 0 <= int(begin_min) <= 59
          assert 0 <= int(end_min) <= 59
          assert begin_hours <= end_hours
          if begin_hours == end_hours:
              assert begin_min < end_min
      except (ValueError, AssertionError):
          raise ValidationError('Make sure the time is in correct format: "hh:mm - hh:mm"')
docs: Makes sure the form data is in 'hh:mm - hh:mm' format and  the start time is less than end time.