3DS:RTC black screen bug

3DS:RTCBUG

The real-time clock (RTC) black screen bug refers to an issue where the console may turn on to an extended black screen, usually right after custom firmware is set up. This is due to the console's raw RTC being reset to the current date. Formerly called "lumabug" when it was thought to be an issue with Luma3DS before the real cause was found out.

Technical details

This issue is due to the 3DS having two clocks, as follows:

Real-time clock (RTC): The RTC is embedded into the system's hardware. It starts at 2000-01-01, and begins ticking upward when the console is powered on for the first time, after which it works even while the 3DS is powered off. This clock cannot be changed without CFW (unless the battery is removed, but that only resets it).
HOME Menu clock: A software value, set as a constant offset from the RTC. Most applications use this to determine the time, and use the offset to determine when the time is changed.

During our guide's Finalizing Setup, we set the real time clock to the actual time, and set the offset to 0 - this helps homebrew apps that use the time, as they cannot access the HOME Menu clock but would be using extremely old dates if the RTC were not changed.
This RTC fix has the side effect of causing the extended black screen on boot.

The database for Play Coins, gamecoin.dat, stores the date of your last earned Play Coin as an RTC date. When you earn a Play Coin for the first time after fixing the RTC, a gap of (usually) ~22 years is created when the 3DS stores the new Play Coin's date. When the system is next powered on after this, it detects the gap and starts listing every single date up to the new Play Coin's date.

This causes the system to need several minutes to boot.

If you leave your 3DS idle for multiple years, then get a Play Coin, this same issue can happen on stock firmware. However, it will take less time to boot, as there are fewer dates to list than the full 22 years caused by an RTC fix.

Is this my issue?

While experiencing this bug, you cannot boot into the HOME menu normally, but you should still be able to access the Rosalina Menu (L + D-PAD DOWN + SELECT) on the black screen, and be able to hold buttons while powering on to boot to GodMode9 (START) and the Luma3DS configuration menu (SELECT).

If your issue has any of the following, it is not lumabug:

If the power LED does not stay on when powering on the console, you are most likely missing boot.firm from the SD root.
If you cannot access the Rosalina menu from the black screen, this is not the RTC bug but may be one of these related issues (the "My console gets stuck on a black screen with blue power light staying on" section).
If you cannot boot into GodMode9 or the Luma configuration menu, you may have missing CFW files or issues with SD integrity. If not, then you likely have a serious issue that will require guided assistance and should ask the Nintendo Homebrew Discord for help.

Fixing the RTC bug

If you have verified that your issue is specifically the RTC bug, the steps for fixing it are as follows:

Power off the console
Take out any game cartridge, but leave the SD card in
- The cartridge is unrelated to this issue, but could also cause issues with booting if it's not being read correctly
Power on the console
Leave the console open and powered on for 10-15 minutes. Do not touch the console during this time

If the console boots successfully in that time, the bug is now fixed and is unlikely to happen again. If the console still fails to boot to home menu, ask the Nintendo Homebrew Discord for more help. Mention that you have already tried the luma black screen process.