I don't know if the problem would exist if I had specified an external time server.
I've just experienced this same problem over the weekend since the UK clocks went forward 1 hour for BST.
I am running Kodi v17.6 on LibreELEC v 8.2.3 with Tvheadend v4.2.5, with the pvr.hts client on the same machine.
I have not touched the LibreELEC time server settings, so it's using the standard internet time server that ships in the LE settings by default.
On Saturday (when we were on GMT), I was looking ahead in the TVGuide as I wanted to schedule a program that started at 07:30PM on Monday 26th March. However, the TVGuide was showing the program as starting at 06:30PM. I then realised that this was probably because the clocks were changing Saturday night/Sunday morning, so I didn't worry.
However, on Sunday morning, following the Daylight saving changes, I went to look in the TVGuide again and everything was still out by an hour including the programs currently playing. The system time, which is also displayed top-right of the TVGuide had been changed to reflect the DST, so this was correct. However, the TVGuide data was all out by an hour e.g. a show which started at 11AM was actually showing as having started at 10AM, so the timeline which goes vertically down the screen was therefore over whichever program would be playing in 1hr time.
To correct this I went into Kodi Settings -> PVR & Live TV -> General -> Clear data. The database was cleared and refreshed from the Tvheadend backend (as mentioned, this is on the same machine). Following this, all the times were then correct. It would be nice if we didn't have to do this whenever the clocks change.