They must have some tolerance i.e. you couldn't cross that point 5 mins earlier than asked because they have given a time to be at YYYYY waypoint . They are obviously doing it for traffic management . Recently we have been given some major speed reduction which make the time impossible so have had vectors to make the time . If there was a plus or minus 2 mins that would be ok but in the terminal area or approaching it , 4 mins maybe too much . I am happy to comply as i don't have the big picture . Just a point to ponder .