In EzekiEl’s opening words, he dates his writing as 30 years from some unknown starting point. Nobody knows what that starting point is.
Most translators believe that has was probably referring to his age at the time that he started his prophesying.
Barnes’ Notes on the Bible comments:
‘Some reckon this date from the accession of Nabopolassar (father of Nebuchadnezzar) in 625-BCE, and suppose that Ezekiel here gives a Babylonian date as a Jewish date (as in Ezekiel 1:2); But it is not certain that this accession formed an era in Babylon, and Ezekiel does not elsewhere give a double date, or even a Babylonian date. Others date from the 18th year of Josiah, when Hilkiah discovered the Book of the Law (supposed to be a jubilee year): This would give 594-BCE as the 30th year, but there is no other instance in Ezekiel of reckoning from this year.’
There is also, of course, the possibility that there is a textual corruption here, and it didn’t originally read 30th. However, this is just speculation.
So what is the answer?
We have no idea, but it doesn’t seem to affect Ezekiel’s message.