Let's take flight BA0015, from London to Syndney, via Singapore. This is data from FlightStats.
The first leg has this data:
"carrierFsCode":"BA", "flightNumber":"15", "departureAirportFsCode":"LHR", "arrivalAirportFsCode":"SIN", "departureDate":{ "dateUtc":"2019-08-17T20:30:00Z", "dateLocal":"2019-08-17T21:30:00" }, "arrivalDate":{ "dateUtc":"2019-08-18T09:45:00Z", "dateLocal":"2019-08-18T17:45:00" },
It takes off from London, on 2019-08-17 at 20:30 UTC and lands in Singapore, on 2019-08-18 at 09:45 UTC
but then the second leg:
"carrierFsCode":"BA", "flightNumber":"15", "departureAirportFsCode":"SIN", "arrivalAirportFsCode":"SYD", "departureDate":{ "dateUtc":"2019-08-17T11:25:00Z", "dateLocal":"2019-08-17T19:25:00" }, "arrivalDate":{ "dateUtc":"2019-08-17T19:10:00Z", "dateLocal":"2019-08-18T05:10:00" },
It takes off, in the past, from Singapore on 2019-08-17 at 11:25 and lands in Sydney on 2019-08-17 at 19:10.
Is is possible that the day doesn't get incremented because the second leg still belongs to the same flight code?