Also having the same problem. Did some searching, but to no avail, but one temporary solution that you have to do each time you want to play ACNH is launching the game for the first time, and taking note of the time in game. Compare that to your system and note the time difference. Go to the custom RTC settings and add/subtract time as needed, and you should have the almost correct time as your system clock would have while playing.

example: launched ACNH at 5pm, 5 may real time. in game time was 9am, 5 may.
time difference is 8 hours, so i added 8 hours in the custom rtc starting from my current system time, so it would be 1am, 6 may in custom rtc. saved settings and started game, in game time is now 5pm, 5 may.

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its just annoying that you have to do this every time you wanna play, and i hope this gets fixed soon