jamclark:
Most reigns start in the civil year (fall, as in 6 months after the scriptural year). The months before generally don't count and is called the ascension period.
I have already determined mathematically that reigns in Judah were based on the civil calendar (i.e. Tishri-based) and that reigns in Israel were based on the the sacred (not 'scriptural') calendar (Nisan-based). (Claiming that both used Tishri-based years would result in anomalies similar to those introduced by the Watch Tower Society's claim that both used Nisan-based years.) Additionally, comparison of various verses indicates that the Jews counted the accession (not 'ascension') period as part of the reign prior to Babylonian influence. (It should also be noted that the Jews had different names for the months prior to Babylonian influence.)