I hope that Iam missing something simple.
I have been unable to change the time of day that a session begins.
The "Change Environment" window changes the time while in surveyor, but
that disappears when driving the session.
The time dial in the Environment Tools is only used to change the lighting and weather conditions at set times during the day. See How_to_Use_Environment_Tools
Your post did not indicate which version of Trainz you are using but here are some other options using the Session Editor:-
the QuickDrive Rule as mentioned by Stagecoach
the Startup Options Rule has a time setting
the Time and Rate Rule has a time setting
If you are using Trainz Plus or TRS22PE (I am not sure about TRS22 standard) you can change the time "on the fly" in Driver during the running of a session by opening the Session Options window and using the time dial to change the time (but this is not very precise) and the date (i.e. season) as well.
I set my environment up in a base-session using the Edit session rather than through UDS which is really the old Quick Drive with a save-mode that can mess things up if we're not careful.
I don't know which is more useful in various problem solving situations - the program build number (e.g. 123794 which then has to be translated into a program name e.g. Trainz Plus) or the program name (which does not tell you the SP/HF number or the build).