Musar for Sanhedrin 193:8

הנוגע במת … הוא יתחטא בו ביום השלישי . This verse contains a clear allusion to the fact that even at times when the ash of the red heifer is not at our disposal man can purify himself by cleaving steadfastly to Torah. If he studies Torah in order to in turn instruct others, to perform the מצות he has studied, to keep all the laws that he has been in the habit of observing, he will certainly achieve purity. Our sages said that "a single day in the calendar of G–d is equivalent to one thousand years in our calendar" (Bereshit Rabbah 8,2). What they intended to convey was that the seven days during which G–d created the universe, refer to a period of six thousand years of regular existence and one thousand years during which the earth would be desolate, destroyed. This is another way of alluding to a period which is totally like the Sabbath, during which no progress of a physical kind is made. Each of the six days indicates one thousand years, as Nachmanides explains at length in his commentary on Parshat Bereshit (compare page 31 Ramban, edition Rabbi Chavell).