Richard Kallos will present "Logical Physical Clocks and Consistent Snapshots in Globally Distributed Databases" by Kulkarni et al. https://cse.buffalo.edu/tech-reports/2014-04.pdf
This paper introduces Hybrid Logical Clocks (HLC). They were originally presented as an alternative to Google's TrueTime, a proprietary timekeeping mechanism in Spanner DB based on atomic clocks for totally ordering transactions. HLC works well for partially ordering events in a distributed system, as well as rooting them in wall-clock time, something that Lamport clocks, vector clocks & co. don't do. Several distributed databases have implemented Hybrid Logical Clocks, such as MongoDB, Couchbase, CockroachDB and Yugabyte.