According to the Watch Tower Society, Ahaziah (of Judah) reigned for a single year, in 906BCE. The Watch Tower Society also claims that Judah and Israel both used Nisan-based dating for the start of reigns.
According to 2 Kings 9:29, Ahaziah's reign began in the 11th year of Jehoram (of Israel). But according to 2 Kings 8:25-26, Ahaziah's reign began in Jehoram's 12th year. Of course, if Judah and Israel were both using Nisan-based dating, Ahaziah's single year of reign couldn't have started in both years. To skirt around this problem Insight weakly claims that Ahaziah was "perhaps anointed to kingsip in c. 907 B.C.E."
But that is not the only problem with the Watch Tower Society's chronology for this period. They claim that Jehu began to reign in 905 BCE, but that his reign is counted from the year after that (904 BCE). The problem (for them) here is that they have Ahaziah's single year and the final year of Jehoram (of Israel) in 906BCE. 2 Kings 8:25; 9:12-13 indicates that Jehu was anointed in Jehoram's 12th year (his final year), while Jehoram and Ahaziah (of Judah) are still alive. Whilst there is some latitude for claiming an accession period for Jehu during Jehoram's final year, there's no reason at all why the following year (905 BCE) would also not be counted as a regnal year.
2 Kings 11:3 also indicate that Athaliah reigned for 6 years. However, JW chronology has her reign lasting 7 years.
Hence the Watch Tower Society adds a spurious year for both Athaliah and Jehu, which is just one of several additions to the divided monarchy to pad out their false chronology.
In reality, Judah used Tishri-based dating and Israel used Nisan-based dating for kings' regnal years, and the author of 2 Kings always counts the accession period as part of the reign. With that in mind, 2 Kings 8:25-26 and 2 Kings 9:29 indicate that Ahaziah's reign began some time after the start of Tishri (late 841BCE) but before the following Nisan (early 840BCE). Jehu was anointed in the same year, and murdered Jehoram and Ahaziah (2 Kings 9:22-29) prior to Tishri (840BCE). Athaliah then began to reign in Judah, still prior to Tishri of 840BCE.