measuring elapsed time when voltage reaches specific value in RC analysis
this statement measures the time constant, triggered on the falling edge.  that is a bit more interesting problem:
.meas TRAN TAU TRIG v(in)=2.5 TD=0 FALL=1 TARG V(out)=(1-0.6322)*5 TD=2m
When run in the schematic, it reports:
tau=0.000319437 FROM 0.00500015 TO 0.00531959