This simple DataType is a DateTime used to define Coordinated Universal Time (UTC) values. All time values conveyed between OPC UA Servers and Clients are UTC values. Clients shall provide any conversions between UTC and local time.
UTC has the concept of leap seconds. Leap seconds can lead to repeating seconds. Therefore applications are allowed to use TAI (International Atomic Time) instead of UTC in any place where UtcTime is used. Details on time synchronization are discussed in OPC 10000-6.
It should be noted that the SourceTimestamp (see OPC 10000-4) and ServerTimestamp (see OPC 10000-4) may originate from different clocks that have no synchronization. It is also possible that one may use UTC while the other uses TAI.