In a networked game, you typically need to keep a single concept of logical time, at which game events happen. The physical clocks of the client and server machines are likely set to different values, so you need to keep an adjustment value to get to logical time from physical time.

I'm assuming that client and server attempt to keep one synchronized time (call this logical time). This logical time is a measurement of how far the simulation has progressed since start (or some assumed start, sometime back in time).

For a simple implementation of this, the client can add its physical time (the actual clock) to each outgoing packet, and the server, when receiving the packet, can remember that value, as well as the server logical clock at time of receipt. It puts both those values, as well as the server logical clock at send, in the next packet to the client. The client can then use these values to calculate both RTT (ping) and offset from its physical time to get logical time.

Client Logical Time = Client Physical Time + Client Offset to Logical Time

  CP1 = clientPhysicalClock()
  send( CP1 )

  recv( CP1 )
  SL1 = serverLogicalClock()


  SL2 = serverLogicalClock()
  send( CP1, SL1, SL2 )

  recv( CP1, SL1, SL2 )
  CP2 = clientPhysicalClock()
  ServerProcessingTime = SL2-SL1
  RTT = CP2 - CP1
  OneWayPing = (RTT - ServerProcessingTime)/2
  ClientOffsetToLogicalTime = ((SL2+OneWayPing-CP2)+(SL1-OneWayPing-CP1))/2
You might want to smooth your estimation of the actual offset, to avoid too abrupt changes, and too much jitter because of a single delayed packet.

There are fancier way to calculate client/server time synchronization; for example, look at the way that NTP calculates time.