What we use and what is legal are 2 separate things.
We cannot use a rate of climb/descent. We cannot say 'be at or above/below/level in X minutes'.
We can issue a time clearance off the clock, but a time check must be simultaneously issued using quarter minutes.
ie:
'ABC123 climb and maintain FL350, be at or above FL330 by one two three zero (1230), time now one two two seven and three quarters (12:27:45)'
That is the only legit US ATC way.