Software development doesn't work like that. Neither does hardware development, for that matter. Companies may have internal target dates for release, but sometimes those dates have to be adjusted due to issues found during widespread testing. At this stage of the game, the public knows to expect new versions of iOS and MacOS to be announced in June at WWDC, with an iPhone announcement usually coming in September. The iOS release usually coincides with the iPhone release, but Mac OS has always been more of a "we'll ship it when it's ready" timeframe. 2020 has been an odd year for everyone, so I wouldn't hold any company to a strict schedule now or for the foreseeable future.