The end-times decides how much of the information in the different sessions that shall be taken into acount when generating the weather-forecast.
For example, if your qualification goes on for 15 minutes, you might want to set the endtime for qualification to 15 minutes, otherwise you will get a forecast of a bigger part of the weahter than will occure in the session.
If the weather is set to time-scaled, in the race-session, the whole weather-file
will be used in the race but at a faster rate than is specified in the weather-file.
So for race-sessions with timescaled weather, use the default endtime.