The Jewish secular year starts at the beginning of the month of Tishri (it's the seventh month of the sacred year), which runs from the middle of September to the middle of October. The beginning of the Biblical year is counted as starting from that point by the WTS. It explains why the service year starts in September and not in January and why the Annual Meeting is held the first weekend in October.
Know Jehovah book chap. 15 p. 286 The Watchman Lives to Get the Report ***
The time would come out the same if, as some do, we count year twelve from the Jewish New Year, Tishri 1, of 607 B.C.E.