Thief Simulator: Play Christmas/Halloween events (without breaking other apps)

General Info

This guide was first published on Steam, but I'm the author.

As many may already know, you can play the Halloween content (mainly the abandoned house being spooky and pumpkin/candy collectibles) or the Christmas content (Sleight and presents) by changing your computer's clock.
Halloween event runs from October 1 to November 30
Christmas event runs from December 1 to January 31

Changing the computer clock's date can lead to other issues tho, particularly if done so for an extended period (like a hour or two gaming). Windows update may muck up, tho it's just complaining, no lasting issues. What was more serious to me is.. If you use a proper browser, all HTTPS connections will fail, because your date is out of sync. This means no website using HTTPS will run.
I like to watch podcasts on my second screen, but youtube ceases to function because youtube uses HTTPS. Same for checking something on the wiki or just googling.

If you only and exclusively run the game, have no second screen, never alt-tab out of it etc, I strongly recommend you to just change your computer's date. Way less hassle that way.
But if you don't, you may want to continue reading.

How To

There is a way to change the date for just the game and not the rest of the Computer.

First download RunAsDate (for x64)
Once you downloaded it, extract it to the game's main directory. This is easiest to find by right-clicking the game in your library and going to "manage" -> "Browse local files", which should open that directory.

Doubleclick "RunAsDate.exe", point it to the thief.exe and select a random date in the date range mentioned above. Make sure you check "Move time forward..." and "Immediate mode"

RunAsDate GUI

That's it. Just hit run.

What about achievements tho

You may notice that when you run the game this way, it runs without steam overlay and steam achievements won't be tracked.

I feared I may not get the achievements, but next time I started it through steam and loaded my profile, I got them.

Just to be sure, you may want to backup your save beforehand and do it in small increments (one achievement at a time for example), I did it that way.

To backup your save, go to "C:\Users\\AppData\LocalLow\Noble Muffins\Thief Simulator" and just copy the whole Profile_0 (or 1 etc depending on which profile you use) away.
That way, you could always try doing it the system date change way if something didn't work.
But as said, it all worked like a charm for me. But better save than sorry.