Rattigan350:
It was 587 BC, the 18th year of Nebuchadnezzar. Some parts of the Bible talk about the 19th year, some the 18th year. The confusion comes about because the difference of time systems, for instance, if you count from the accession year, when the king came to power, versus when his regnal year began starting on the next Nissan 1.
For instance, Nebuchadnezzar's accession year was in 605 BC. His father died in August and he ascended the throne shortly thereafter. However, his first regnal year did not begin until March/April 604 BC.
Things might also get muddled since the pre-Exile Jews evidently went from Tishri to Tishri (September/October), while post-Exile Jews adopted the Babylonian convention of Nissan to Nissan.
From Babylonian sources, it is evident that Jerusalem was destroyed in Nebuchadnezzar’s 18th regnal year, which stretched from March/April 587 BC to March/April 586 BC.
For those that insist on 586 BC, I don’t make too much an issue. It’s only a year, unlike the 20 year gap in Watchtower chronology.