This night we get another leap second, meaning that the last second of today is not 23:59:59 but 23:59:60 instead. The last time this happened was on June 30, 2012 and lead to issues on several servers due to bugs in the Linux kernel. Since then these bugs were fixed and also methods of hiding the leap second altogether were implemented. Here you can find a good overview over different configurations of kernel, ntpd and tzdata and how a leap second will be handled using these configurations. What is missing here is how a setup using systemd-timesyncd instead of ntpd will handle this situation.
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