The surveying on Washington started in 1791; the federal government moved there in 1800. That gives the current year in Stranger as somewhere between 1994 and 2003.
A very early chapter says that the second human expedition to Mars, which found Smith as the sole survivor of the first expedition (born there and raised by Martians), departed a quarter century after the first. That puts the first human expedition at sometime between 1969 and 1978, or between 8 and 17 years after the novel was published. Since the first expedition was four married couples, Heinlein is envisioning really rapid advances in space technology, much more rapid than the real world attained. Not to mention a third world war sometime not long after the first expedition, and the creation of a world government with its own police and military after that. By now the main events of the novel are more than a decade in the past.