Charts not updating on PiHole dashboard until rebooted

I solved this issue so am not posting in the Help section, but there seems to be a problem with PiHole's charts in the dashboard.

I changed the extension cord to my routers/PiHoles this morning and when the power came back up everything seemed to be working and Pi-Hole (v5.1.1) was reporting Total Query numbers but the charts were blank, showing a one-hour time range but no bars. I thought maybe it look an hour to update or so but many hours later it still was showing blank charts but ever-increasing client query counts in the boxes at the top of the dashboard.

After some googling I found recommendations to clear the browser cache and/or Ctrl+F5 on the page. Neither of these made any difference. I was using Chrome so I fired up Firefox instead and again the charts were blank on the logged-in dashboard, but the KPI boxes at the top were still counting up. So this seems to be a server-side issue and not a client browser/cache problem as it occurred on two different browsers.

I found another post by someone reporting that after a cold boot the charts hadn't appeared. After running a 'sudo shutdown -r now' on the PiHole Raspberry Pi (B 1) it came back up again and the charts were working in both browsers again.

I thought the developers might like to know and that this might be useful for others hitting the same issue. Note that it's possible that when I first turned the power back on this morning the PiHoles cam up before the Internet connection/routers they were connected to, which might be a reason PiHole got stuck in a funny mode with no charts, perhaps.

Did you do a controlled shutdown of the Pi, or just pull the power?

Thanks for the response, jfb. I did attempt shutdown safely with a sudo shutdown -h, which I believe ran to completion before the power was cut. I generally do this if I can, although over the last several months that I've been running PiHole (one on a Pi B Model 1 and later a second on a Pi Zero (1.1 I think)) I've had the Pi B become unresponsive on three occasions, which has required that I pull its plug (which is why I set up the Zero as a backup DNS server).

If this happens again, please check the time on your Pi-hole machine.

Charts may also not display correctly if there is a time difference between the browser and the Pi-hole machine.

RPis lack a dedicated RTC, relying instead on time servers for a correct time. (click for more)

Until a sync with the time servers succeeds, an RPi will continue to run from the time it has read during boot from a file created by fake-hwclock.
On Raspberry Pi OS, that file get's written to every hour and on a controlled shutdown.

On occasions, time-syncs won't work at all.

If time is off by too much then, you may set your time manually, e.g. by

timedatectl set-time '2020-11-02 10:00:00'

Thanks, Bucking_Horn - I'll make sure to do that. I can see that if the Pis came up before the Internet connection did then they may have failed to sync their clocks.

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