Every reporting carrier as defined in § 234.2 of this chapter and any carrier that voluntarily submits data pursuant to § 234.7 of this chapter shall file, on a quarterly basis, the information specified in BTS Form 251. The reporting basis shall be flight segments originating in the United States. The reports are to be submitted within 30 days after the end of the quarter covered by the report. The calendar quarters end March 31, June 30, September 30 and December 31. “Total Boardings” on Line 7 of Form 251 shall include only passengers on flights for which confirmed reservations are offered. Data shall not be included for inbound international flights.