In addition to Daniel's prophecy of the 70 weeks, there is also the prophecy of the 7, 62 and 1 week.
For this we take - just as with the prophecy calculations concerning the 70 weeks - the necessary known historical data as a starting point for calculating the fulfillment of this prophecy.
Since the results correspond to the facts, the method of calculation used may be considered correct. And the correctness of these outcomes, in turn, lends credence to the calculations of prophecies pertaining to the Báb and Bahá'u'lláh.