By default, the UTC Time parameter is set to false. The Flight Status or Historical Flight Status API will detect and pass the information on all the flight departed (Scheduled, Estimated, Actual) on the date based on local time.
It is common for user to see 2 flight IDs for the same flight number when the Flight Scheduled Departure time (Local) is close to midnight (I.E. 2345Hr). Small delays may resulted in Actual/Estimated Runway Departure time to crossover to next day (I.E. 0000Hr).
Flight Status queries by departure (or arrival) date will be found if any departure time (or arrival time if querying by arrival) is on that given day, including: scheduled, estimated, actual, gate, and runway times.
If interested in Flight departure on the date itself, please consider setting UTC Time to true.