What's your go to way of figuring out how old things in your world are?

This can be anything from how old is the oldest tree in this forest to what year is it in this world, and also anything in between like the last time this event happened or how long a certain civilisation has been around. I'm using this for world building and wanted to know what methods you guys use for your worlds.<br><br>Currently I'm using the Fibonacci sequence for most things like events and ages of cities/towns.