I can go to any encyclopedia and prove that Jesus Christ was not born on December 25th. This is the line often used by Jehovah's Witnesses to point out that Christmas is NOT a Christian holiday. Yet, those same encyclopedias cannot be trusted when it comes to the date of Jerusalem's desolation because they all disagree with the Watchtower's date of 607 B.C.E..
Think of it this way. Christian who learns that Christ was not born on 12-25 goes and tells his preacher this new found information. Adament preacher insists that it is in spite of all contrary evidence (those encyclopedias). Witness who learns that 607 is not the date of Jerusalem's desolation and goes and tells the congregation elders this new found information. Adament elders insist that it is in spite of all contrary evidence (those same encyclopedias).