potentially wrong handling of GTFS Time values

[derhuerst]

GTFS Time is not defined relative to midnight, but relative to noon - 12h. While that makes "writing" GTFS feeds easier, it makes processing a lot harder.

Expected functionality

As explained in my note about GTFS Time values, with the Europe/Berlin time zone (+1h standard time to +2 DST shift occurs at 2021-03-28T02:00+01:00), I expect

• the departure_time of 00:30 of a trip running on 2021-03-28 to happen at 1616884200/2021-03-28T00:30+02:00, not at 1616887800/2021-03-28T00:30+01:00;
• the departure_time of 06:30 of a trip running on 2021-03-28 to happen at 1616905800/2021-03-28T06:30+02:00, not at 1616909400/2021-03-28T06:30+01:00.

Describe the bug

I'm not familiar with this code base (just a random stranger dropping by to poke around 🙈), but it seems that combine_gtfs_feeds is affected by this problem on those days that the DST <-> standard time switch occurs on.

I'm not entirely sure how that actually manifests in combine_gtfs_feeds' output, but I assume that wrong headway-based stop_times will be calculated.

I tried to find some places in the code base:

• combine_gtfs_feeds/combine_gtfs_feeds/cli/run.py

  Lines 106 to 112 in cb78603

  	def convert_to_seconds(value):
  	"""
  	Converts hh:mm:ss format to number
  	of seconds after midnight.
  	"""
  	h, m, s = value.split(':')
  	return int(h) * 3600 + int(m) * 60 + int(s)

• combine_gtfs_feeds/combine_gtfs_feeds/cli/run.py

  Lines 150 to 168 in cb78603

  	stop_times_update['arrival_time_secs'] = stop_times_update[
  	'arrival_time'].apply(convert_to_seconds)
  	stop_times_update['departure_time_secs'] = stop_times_update[
  	'departure_time'].apply(convert_to_seconds)
  	stop_times_update = stop_times_update.loc[
  	stop_times_update.index.repeat(
  	stop_times_update['total_trips'])].reset_index(drop=True)
  	stop_times_update['counter'] = stop_times_update.groupby(
  	['trip_id', 'stop_id']).cumcount()
  	stop_times_update['departure_time_secs'] = stop_times_update[
  	'departure_time_secs'] + (stop_times_update[
  	'counter'] * stop_times_update['headway_secs'])
  	stop_times_update['arrival_time_secs'] = stop_times_update[
  	'arrival_time_secs'] + (stop_times_update[
  	'counter'] * stop_times_update['headway_secs'])
  	stop_times_update['departure_time'] = stop_times_update[
  	'departure_time_secs'].apply(to_hhmmss)
  	stop_times_update['arrival_time'] = stop_times_update[
  	'arrival_time_secs'].apply(to_hhmmss)

---

related: google/transit#15

[stefancoe]

This library does not make any changes to the schedule that is published in the input GTFS feeds. In other words, the output stop_times.txt reflects what is in the input GTFS feeds in the output, for the trips that are kept.