tie jitter function

Hi,

I wonder if there's function or expression based on existing functions to check tie type of jitter. To elaborate, the goal is to check corresponding edge  difference from clk_sim (from simulation) and clk_ref (based on ideal clock). 

The expression of jitter should be something like: jit = cross(VT("/vout_sim") 0.5 1 "rising"  t "time"  nil ) - N*Tref, in which cross() gives edge of clock_sim, Tref is known (1/fref), but as you can see, I need expression for N.   I applied xval() function to cross() result, but it still gives rising edge time, instead of the index of the clock edge.

any idea?

thanks,

Kevin