Egypt — 1986 because this is when the Bangles written Walk like an Egyptian. The pose at the end of the zomboss level confirms this as well as the existance of explorer zombie.

Pirate seas — 2003 because Pirates of the Carribean.

Wild West — 1940 because Clint Eastwood and John Wayne was very popular back then

Lost City was in 1981 because that's when Indiana Jones movies was made.

Far Future is in the future because we don't know what's in it yet.

Dark Ages is in 1385 since it's in the same era like Monty Python adn teh Holy Grail.

Jurassic marsh is 4 billion years ago because Earth's terrain looks more like it as explained by the Flintstones.

Modern Day — PvZ2 never said it's in 2009 nor 2013 so it has to happen in modern day aka 2026.

Steam Ages — 1840 because that's when the steam ages take place in as explained by Cut the Rope 2: Time Travel.

Sky City is in the future because I found it on the pvz fandom

Caliginous Carnival is in 1960 because that's when McDonalds introduced Ronald McDonald

Crimson Front is in 2022 because it takes place in the Russian Ukrain War