Here is a complete timeline of all events that take place in When Worlds Collide.

Date (MSC) Planet/Earth (Earth IDs) Description
January 19, 0 All of them, including Non-Earths The Great Merge occurs
February 2, 2 Great Detroit (E-III) Groundhog Day Massacre; Stimpson Cat dies.
February 6, 2 Great Detroit (E-III) The Reich Resistance forms.
March 11, 5 Wolfenstein (E-I) ; Danville (E-II) Wolfenstein invades Danville successfully; Dr. Heinz Doofenshmirtz dies.
July 4, 6 Danville (E-II) The Danville Resistance forms.
September 1, 7 Danville (E-II); Great Detroit (E-III) The Allied Resistance forms.
July 17, 8 Great Detroit (E-III) Ren Höek dies from old age.
August 4, 9 Wolfenstein (E-I) ; Atoll Atoll is invaded by Wolfenstein.
September 3, 10 Danville (E-II) ICENE forms.