The new "Agent-Server Clock Difference" metric for the RHQ Agent will give negative numbers if the agent's clock is slower than the server's clock.

However, the Monitor/Graphs tab will not show negative numbers at all (see attached screenshot).  I found this by testing rhq-915.  I first set the agent clock slower than the server's, waited for some measurements, then set it faster than the server's (2 minutes slow and fast).  

What happens is the graph still bottoms out at 0, but if a measurement period contains negative measurements, it shows the bar descending below 0, to the bottom of the image.