As described in 1512

All timestamps etc. in OPC UA are UTC. However, in UTC there are leap seconds which lead to repeating seconds when a leap second occurs. This disrupts the order of events and measurements during those two seconds. Also the aggregates become inaccurate since some days have a second more. As the leap seconds come unregularly depending on the sun it would be desirable to get a mechanism in place where applications can get the information on the UTC as well as the TAI (similar to UTC without leap seconds) and the leap seconds as difference between UTC and TAI). This could for example be achieved by either transmitting UTC or TAI and in addition the difference of leap seconds (currently about 34 seconds).

Solution is to state that it is allowed to use TAI in the UTC time bucket. This needs be be defined in the data type in part 3 in addition to the general description on time sync in part 6.
The solution was discussed and agreed on in the F2F meting in Foxboro 2011.