January as 0 seems like a very definsible choice. Presumably they did it to match C. If anything the WTF there is that the day of the month doesn't start from 0.

Dunno what's up with getYear() though!

getYear() must have been written in the 1990s during the early years of Java:

   95 = 1995
   96 = 1996
   97 = 1997
   98 = 1998
   99 = 1999
  100 = 2000 !?

Wasn’t Java 8 released 11 years ago? If PHP had been in its current state back then it would have been a masterpiece.