System clock miracoulously jumped ahead by one hour overnight

[Karl Kastner]

The clock miraculously jumped one hour ahead on my system overnight. Two nights ago there was the switch to summer time. This went fine, i.e. the UTC system clock was not changed and the system just added another hour to interpret it as UTC+2 (CEST). However, this morning I found that over the next night the UTC system clock was incorrectly advanced by one hour. The system was suspended, but not rebooted, there is no Windows installation/virtual machine that could mess with the clock. ntpdate -q displays the error message "no servers can be used, exiting", but this cannot explain why the clock jumped ahead. The clock was reset correctly after I replaced ntp with chrony.

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: tzdata 2018i-0ubuntu0.18.04
ProcVersionSignature: Ubuntu 4.10.0-42.46-generic 4.10.17
Uname: Linux 4.10.0-42-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.6
Architecture: amd64
CurrentDesktop: GNOME
Date: Mon Apr 1 10:17:50 2019
InstallationDate: Installed on 2015-11-05 (1242 days ago)
InstallationMedia: Ubuntu 14.04.3 LTS "Trusty Tahr" - Beta amd64 (20150805)
PackageArchitecture: all
SourcePackage: tzdata
UpgradeStatus: Upgraded to bionic on 2018-05-24 (311 days ago)