Add an optional clock_offset property to sparks.

Useful for syncing client clocks to server time.

index.js

@@ -3,21 +3,47 @@ module.exports = {
   server: function (primus, options) {
     primus.Spark.writable('latency', 0)
 
-    primus.transform('incoming', function (packet) {
+    if (options.use_clock_offset) primus.Spark.writable('clock_offset', 0)
+
+    primus.transform('incoming', options.use_clock_offset ? setBoth : setLatency)
+
+    function setLatency(packet) {
       var latency = packet.data._latency
       if (latency && typeof latency == 'number') {
         this.latency = latency
         return false
       }
-    })
+    }
+
+    function setBoth(packet) {
+      var latency      = packet.data._latency
+        , client_clock = packet.data._clock
+        , cont = true
+
+      if (latency && typeof latency == 'number') {
+        this.latency = latency
+        cont = false
+      }
+
+      if (client_clock && typeof client_clock == 'number') {
+        this.clock_offset = Date.now() - client_clock - Math.floor(this.latency / 2)
+        cont = false
+      }
+
+      return cont
+    }
   },
 
   client: function (primus, options) {
-    primus.on('incoming::pong', sendLatency)
-    primus.on('incoming::open', sendLatency)
+    primus.on('incoming::pong', options.use_clock_offset ? sendBoth : sendLatency)
+    primus.on('incoming::open', options.use_clock_offset ? sendBoth : sendLatency)
 
     function sendLatency() {
       this.write({ _latency: this.latency })
     }
+
+    function sendBoth() {
+      this.write({ _latency: this.latency, _clock: Date.now() })
+    }
   }
 }