Unix epoch (seconds) of the start of a GTFS service day: local noon on the
service date minus 12 hours. Using noon as the anchor keeps results correct
on daylight-saving change days, where "midnight plus N hours" is off by the
shifted hour.
Parameters
serviceDate: string
Service date in YYYYMMDD format (e.g. '20260713')
timezone: string
IANA timezone name (e.g. 'Europe/Paris'). GTFS stop times
are always expressed in the agency's timezone (agency_timezone).
Unix epoch (seconds) of the start of a GTFS service day: local noon on the service date minus 12 hours. Using noon as the anchor keeps results correct on daylight-saving change days, where "midnight plus N hours" is off by the shifted hour.