From WSPR 3.0 User’s Guide we know that computer clock must be correct to within about ±1 second.
I've tried to test it and find practicullar limits of error.
First PC is receiver, and have internet time sinchronisation.
Second PC is tranceiver and I setup clock as I want.
We can compare DT parameter on first PC, and get or get not spots from other WSPR world stations.
Band is 40M. Power is 0.5 W.
Accuracy clock manually setup is near one second.