Key Scripture

Eze 1:1-3

Now it came about in the thirtieth year, in the fourth [month], on the fifth [day] of the month, while I was in the midst of the exiled people by the river Chebar, that the heavens were opened and I began to see visions of God. 2 On the fifth [day] of the month, that is, [in] the fifth year of the exile of King Jehoiachin, 3 the word of Jehovah occurred specifically to Ezekiel the son of Bu′zi the priest in the land of the Chaldeans by the river Chebar, and upon him in that place the hand of Jehovah came to be. 

Key points

30 years previous was the 18th year of Josiah. The 18th year of Josiah is 5 years after the 13th year of Josiah, when Jeremiah started his preaching. Therefore, there is a constant 5 year difference between the 30 years of Ezekiel, in Babylon and the years of Jeremiah's preaching in Jerusalem. The 5th year of Jehoiachin's exile is the 5th year of Zedekiah's kingship.

Quotes from Watch Tower Publications