Thank you for your help guys!
I got it.
I added +1 year (because I want my game to always take place today in a year of time. The +1 on the month was given in Asmodean's example already "Browser.ExecJS("this.myDate.getMonth()")+1" and it makes the month show correctly. I did not quiet get why a 1 needs to be added to the months. But now everything works.
For any who is interested in how I use it in my game "SESA":
Instead of having the start date (which is like your highscore) hard coded, so same every game. it will detect the current date as you play and adds one year to it. So the game will be called "SESA - One Year From Now". Since the prologue tells the player that an asteroid struck earth in the near future and now you have to control the Space Exploration and Survival Administration to colonies your solar system and more to come. I just thought its a cool idea to subtly seed the thought of „what would i do/ who would i be/ would i be if an asteroid struck earth in one year from today. How would i live this year... Just a very small detail and a bit of over engineering, as always. Buts thats just me hey
I hope this helps more people in the future maybe ;)
Thanks again for your help!