An old issue I know and one close to MDWS, Tony's heart, incorrect RPi clocks.
An easy fix I've found on line which I have had to use a number of times on the 3 RPi's I look after running DSLStats to MDWS, one Mk.1 and two RPi Zero W's all running Raspbian.
To my surprise, all my RPi's survived last weeks GMT reset without any issues, reverted to GMT automatically.
Today, after a planned router/modem re-synch my RPi came back post re-boot with over an hour time error, I have read on the RPi forums that if a time error is greater than 100 sec's the Raspbian "ntp" routine can not correct this.
Apply the following fix:
From RPi command prompt:
sudo /etc/init.d/ntp stop
sudo ntpd -q -g
sudo /etc/init.d/ntp start
This will correct the RPi clock and bring it within range of the "ntp" routine which will now update from on line time reference sources.
Other fixes are available on line but this is simple and has always worked for me.