You can count rings of a tree you cut down - if you have a system for relating/storing their planting time deltas then you can measure by cutting down a tree and planting new ones.
Edit: Apparently, together with climate data you can determine the year the tree was cut down hundreds of years after the fact [0].
If you plant a tree that lasts for 10,000 years (is there such?), I wonder if you can use its height or the shadow it casts (trunk thickness) to measure time.
Of course, someone would just come and kill it. Unless you can make it undesirable to get close to it.. for example fill it with thousands of snakes and spiders.
As far as we know, there are a few clonal plant colonies that have survived longer than 10K years. No individual plants have quite made it to 10K, but some have made it past 2-3K years.
>If you plant a tree that lasts for 10,000 years (is there such?), I wonder if you can use its height or the shadow it casts (trunk thickness) to measure time.
You need to make some sort of sign that indicates the age of the oldest trees in the forest is the clock. And make it impossible or unprofitable to log the forest for wood, maybe by building hundreds of annoying stone pillars at inconvenient spacings
