Here's the thing dear fans like us have to understand,
The bigger the game is, there's more bugs to consider about.
More features = More time needed to implement it + Figuring out and fixing the bugs
Therefore deadlines are just an unrealiable way to measure time in game development, or more accurately, in any kind of programming projects.
If you have some spare time to try new things, you may download Unity creator for free, research on Unity tutorials and explore how Unity works!