clGetEventProfilingInfo time stamps not synchronized?

I am trying to profile code in a multi GPU environment with clGetEventProfilingInfo() but it looks to me that I cannot correlate the time stamps from different GPUs. The spec just says, that such a time stamp is “A 64-bit value that describes the current device time counter in nanoseconds […]”. So, do I have to assume that these don’t have to be synchronized?