How to get flight Id for future flights

Hi
I need to get faFlightID, for all future flights which are containing in AirlineFlightSchedulesResult array, I saved result(schedule) from request data which has been sent few days ago. There are past flights and future flights info in this schedule now

I use GetFlightID method and send 2 params, ident and departureTime.
For past flights i get result like this
{
“GetFlightIDResult”: “GLA3008-1427926680-schedule-0005”
}

For future flights i get error

{
“error”: “flight not found”
}

From docs

GetFlightID looks up the “faFlightID” for a given ident and departure time. This value is a unique identifier assigned by FlightAware as a way to permanently identify a flight. The specified departure time must exactly match either the actual or scheduled departure time of the flight. The departureTime is specified as integer seconds since 1970 (UNIX epoch time).

If more than one flight corresponds to the specified ident and departure time, then only the first matching faFlightID is returned. Codeshares and alternate idents are automatically searched.

How can i get faFlightID for future flights? Thanks!

The faFlightID is usually available for a flight when it is less than 48 hours from departure (sometimes only a few minutes from departure), but it will vary depending on whether it is a GA flight or a commercial flight, and whether it is in an area of primary flight coverage (US/Canada/Australia) or not.