Event: Use -1.0 for undefined/unset timestamps

This can happen if either there's no network timestamp associated with
an event, or there's currently no event being dispatched. Using 0.0
isn't great as it's the normal start timestamp before reading a network
packet. Using -1.0 gives the caller a chance to check and realize what's
going on.

NEWS

@@ -24,6 +24,12 @@ Breaking Changes
   a small overhead when enabled. There's not enough users of network timestamp
   metadata to justify the complexity of treating it separate.
 
+- The ``current_event_time()`` builtin function as well as ``Event::Time()``
+  and ``EventMgr::CurrentEventTime()`` now return ``-1.0`` if not timestamp
+  metadata is available for the current event, or if no event is being
+  dispatched. Previously this would've likely been 0.0, or the previously
+  dispatched event.
+
 New Functionality
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