Presumably because I began work on my route sometime around November last year, it seems every time I now open the editor to work on it this is the time of year it defaults to. I’ve tried tweaking dates/times in the Route Definition and Timetable settings so I can play the game in Summer but these do not seem to change the environment, either when playing or working on the route. The only thing that works for me is to use a console command each time I open the editor. I must be missing something obvious and hope some kind person can point me in the right direction. Edit: I've just restarted the editor and it has kept the Summertime date I set using the console last time. I'm very confused now. How I'd love some in depth tutorials.
In the editor, there is a "World Settings" pane (usually it's another tab of your object properties pane on the lower right-hand side of the window). Open your map, change the date/time there and save your map. Afterwards it will forever default to that time you entered there when loading the map.
Thanks fceschmidt. I managed to convince myself that I'd already tried changing that setting but now it is working every time.