Great post. Very simple really. Just use the bible, no hokus pokus. A simple way of looking at this is to just turn to Zechariah 7 :
1 And it came to pass in the fourth year of king Darius, that the word of the LORD came unto Zechariah in the fourth day of the ninth month, even in Chisleu;
2 When they had sent unto the house of God Sherezer and Regemmelech, and their men, to pray before the LORD,
3 And to speak unto the priests which were in the house of the LORD of hosts, and to the prophets, saying, Should I weep in the fifth month, separating myself, as I have done these so many years?
4 Then came the word of the LORD of hosts unto me, saying,
5 Speak unto all the people of the land, and to the priests, saying, When ye fasted and mourned in the fifth and seventh month, even those seventy years, did ye at all fast unto me, even to me?
4th year of Darius rule was in 517 (ascention year was 521). So in this month it was seventy years of fulfillment. So, 517 + 70 = 587. Hope that helps.