As Mark have said, you can buy an RTC. For how many months right now my
standalone (not connected to the internet) Raspberry Pi 4B has been very
stable with time using DS3231 module
https://shopee.ph/DS3231-Mini-RTC-Module-i.18252381.315148783. Sharing to
you as well https://marc.info/?t=159819358900001&r=1&w=2.

Thanks and best regards,

On Sun, Jan 10, 2021 at 12:37 AM Mark Kettenis <mark.kette...@xs4all.nl>

> > Date: Sat, 09 Jan 2021 17:24:59 +0100
> > From: "Filippo Valsorda" <fili...@ml.filippo.io>
> >
> > (Emailing the arm@ list because this is a common issue on arm platforms,
> > although not arm specific. Let me know if I should redirect.)
> >
> > I run a simple OpenBSD firewall on a RPi 4, which doesn't have a
> > real-time clock. When the power goes out, the firewall boots faster
> > than its upstream, so it doesn't have network connectivity in the first
> > seconds.
> >
> > This interacts poorly with ntpd's settime logic: ntpd will only use
> > settime in securelevel 0 (see auto_preconditions() in ntpd.c), and will
> > only try reaching the Internet twice, with a 1s pause, upon starting.
> >
> > The result is that the firewall boots, gives up on settime, and ends up
> > stuck forever with a clock weeks old, enough to break the system, and
> > too far for ntp to catch up.
> >
> > I'm not sure what the right solution is. I think I would want ntpd to
> > wait until it has network connectivity at boot, but I'm not sure if this
> > is something I should hack myself or maybe there's space for an ntpd CLI
> > option.
> >
> > Opinions?
> Add an RTC to the Pi4.  They can be bought for a few euros and can be
> enabled by adding the appropriate device tree overlay to the
> config.txt file on the firmware partition of your boot disk.

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