Indeed the dashboard is aimed at displaying real-time data rather than historical... which does really mean that clocks need to be in sync...
Generally if it's on a local network then a local timeserver is the simplest approach - usually you just need to add a server line to the ntp.conf file- but I see on latest Raspbian for Pi (Stretch) it is now in the file /etc/systemd/timesyncd.conf instead.
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