This is what I learned from the Isaiah books (apologies to those groaning over seeing a portion of it in print):
Book 1, page 253 (highlighting mine):
"It must occur in that day that Tyre must be forgotten seventy years, the same as the days of one king." (Isaiah 23:15a) Following the destruction of the mainland city by the Babylonians, the island-city of Tyre will "be forgotten." True to the prophesy, for the duration of "one king"-- the Babylonian Empire-- the island-city of Tyre will not be an important financial power. Jehovah, through Jeremiah, includes Tyre among the nations that will be singled out to drink the wine of His rage. He says: "These nations will have to serve the king of Babylon seventy years." (Jeremiah 25:8-17,22,27) True, the island-city of Tyre is not sublect to Babylon for a full 70 years, since the Babylonian Empire falls in 539 B.C.E. Evidently, the 70 years represents the period of Babylonia's greatest domination--when the Babylonian royal dynasty boasts of having lifted its throne even above "the stars of God" (Isaiah 14:13) Different nations come under that domination at different times.
If only they could see how these same statements could be applied to the fall of Jerusalem. Of course, that would mess up their 1914 thingy.