I don't know exactly what you mean. But you can change the length of the seasons in the Environments file. The year is always exactly 1 year long. You can also define the season for the beginning of the game. With actions you can set the gametime to 1, then you get the experience to survive the winter.

Sorry should of been clearer.
As you say each year is exactly 1 year long so would it be possible to have an in game year counter that shows how many years you have been playing.
Maybe next to the human count you would have a year count,so each summer it would increase by 1.
after 1 summer the year count would be 1, 2 summers the year count would be 2, 3 summers the year count would be 3, the 4 then 5 and so on so you can see how many years you have been playing any game.

Ingame year counter: The counter already is in the game, but not shown in the ui. I used it for the timed spawn events in my first amazon szenario.

It is also stored in your savegame, but not shown. :-(

If you start in summer the counter should count normally and give you the "survived the winter" achievement in summer.

I configured all seasons with a length of 0.5 once. The map had a nice spring, a nice summer, survived the winter! Yippieh! again, spring and summer. survived the winter.

Maybe this can help in addition:

ActionSetTimeOfYear
Parameters
Value: Decimal Number (default: 0)

http://madrugaworks.com/dawnofman/reference_actions.php

Thanks for reply

Yeah, there's a 'hidden' year counter. It would be nice if the devs included how many years the settlement has been going in the F2 info or something. but I'm guessing they didn't because strictly speaking thousands of years are passing between ages.

It probably should be possible to make something like

Time elapsed =10y hint "10 years passed"

and then copy paste it for 20y,30y,50y etc

Yes, would be an option. Could be part of a scenario. But an ingame counter for survived years would still be nice. Even in the 8-Key tab under "Survived Winters".