The A7M should have been in service in late 1943 or early 1944. A few things got in the way, Mitsubishi, the IJN and mother nature.
1) Jiro Horikoshi, the lead fighter designer at Mitsubishi, started work on the A7M as soon as the A6M was entering service. The IJ had presented specs for the A7 fighter to replace the A6M as soon as the A6M had entered service. This was standard for both the IJN and USN, ordering the replacement as soon as the new aircraft entered service. However, Mitsubishi kept reassigning Horikoshi to the A6M upgrade program which took his efforts off of the A7M program. The A6M upgrade program should have been left to junior Mitsubishi aircraft engineers and Horikoshi should have been kept on the A7M program.
2) The A7M was ready for test flights in late 1943, despite Mitusbishi's interference. Horikoshi wanted to use the 2,200hp Mitsubishi Ha-43.11 engine, but the IJN wished to standardize on the Nakajima Homare 21 that the N1K2-J used. Horikoshi told themit wasn't powerful enough, but they instructed him to proceed with the Homare. Once the redesign work was done, the A7M1 prototype, powered by a Nakajima Homare 22 engine, flew on May 6, 1944. The IJN's comments were that it possessed exellent stability and controlability, but was decidedly underpowered. The IJN then told Mitsubishi to proceed with the originally intended Ha-43.11 engine. When this prototype, the A7M2, was flown (I don't know the date) it was immediately obvious that manuverability, rate of climb and maximum speed were noticably improved, while it retained its excellent stability and control features.
3) An earthquake seriously damaged the Mitsubishi factory intended to produce A7M2s, delaying production by months.
4) USAAF B-29s bombed the factory (we didn't know what was produced there, simply that it was a Japanese factory) further delaying production.
A few A7M2 production aircraft were completed before the war ended, but they saw no combat. Despite the lack of combat, I would almost like to see the A7M2 added to AH as a perk Japanese fighter.