Imported from previous forum
Please also see --> http://www.fixtradingcommunity.org/pg/discussions/topicpost/2893609 --> this describes extending the existing timestamp all the way to nanosecond level precision to support MiFID regulations (microseconds) so yes application layer timestamps (encoded in SBE) such as transaction time could also be in uInt64 format.
FIXP session layer is completely independent from application layer timestamps. Unlike FIXT, it does not impose a header on application messages, and thus, it does not carry sending time as FIXT does. On the other hand, application message designers are free to use any precision and efficient encoding for time.
Will Application messages be changing their Timestamp data type to u64 nanotime? If not, how will the FIX Performance Session Layer nano timestamps interact with FIX 5.x Application message timestamps?