Basically the title, you need to use the skills you have now and be a productive member of society.
I don’t mean go back and show the wheel or try invent germ theory etc.
For example I’m a mechanic i think I could go back to the late 1800s and still fix and repair engines and steam engines.
Maybe even take that knowledge further back and work on the first industrial machines in the late 1700s but that’s about it.
Depends on the skillset in question.
On one hand I work with IT/Clusters and robotics for the geophysical exploration sector. 20 years, probably. Beyond that and it gets dubious apart from this one system that actually runs on MS-DOS to this day (because MS-DOS is surprisingly good at realtime stuff if you want it to do something very simple).
On the other hand I do a lot of digital I/O and automation which would probably be very useful in the 60s, maybe even before if I manage to join the pioneers.
On top of that, I grew up on a dairy farm, and learned a lot of that trade from my dad. I can milk a cow by hand, so if that was all I needed to do, I could go back all the way to Mesopotamia.