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.
I’m a locksmith so any time since the invention of the pin tumbler lock 150ish years ago I will be fine. I don’t prefer it but I can hand file keys without any electric key cutting machines. Before that the bit and barrel locks that were used I know enough to get by though admittedly I don’t know enough history to say roughly how long ago those were invented.