Time expands relative to what?
We can measure the expansion of space, because (despite tommac's protestations) our measuring instruments don't participate in the expansion of the universe, except by moving apart at the galactic-cluster scale and above.
And we can measure differences in clock rates for observers in different locations under SR and GR.
But if time "expanded" for the whole universe, how would we know? Which is, presumably, why we model space expanding using defined, uniform time measurement.

Grant Hutchison