I had the idea that around 18-20 months elapsed from the time they left the safe zone in Boston to the time they arrived back at Tommy's in Wyoming after leaving Salt Lake City.
Obviously Joel won't change much since he is a fully grown man in his late 40s or early-mid 50s, but Ellie is probably around 16 by the time the game ends and she looks the same as she did when the game began.
Joel commented he was "kind of young" when his daughter was born, which would suggest he might have been 16-20 years old at the time, so when she was 10 in 2013, Joel was probably no older than 30, maybe closer to 28 or even 27, which would make him around 47-50 as of 2033 when things start in Boston.
He won't look much different after 18-20 months, but Ellie should probably look less like a child and more like a young woman.
if im not mistaken, the game went from Winter, Spring, Summer, Winter back to Spring, thats only a little over a year, around 14 months.
Ellie said she was 14 years old when they met up with Henry and Sam. so at the most she would be 15 by the end of the game.
As best I can tell, the game starts in the spring/summer of 2033, there's no way it is starting in the winter. It is set in Boston and there's not an inch of snow on the ground and there's no ice anywhere, so it cannot be winter, at least not a Boston winter.
I figure it starts in spring/summer, then it goes winter, spring, summer, winter, spring, so basically 2 full years.
but if thats the case then what happened to Winter after that? if it started in spring/summer then the next season would have been winter but there was no snow or cold as you said there would be. and i doubt that Joel and Ellie just sat around for months doing nothing until winter passed.
either the game started in late winter or early spring.
winter>spring>summer>winter>spring or spring>summer>winter>spring.