Network Time Security (NTS) is a new standard for secure time distribution. NTS was published by ITEF as RFC 8915 in September 2020. NTS is a fairly complex standard with a key establish mechanism separate from the time synchronization.
Netnod Internet Exchange (Netnod), provides time keeping services including NTS, has just published a whitepaper that gives a good explanation on how NTS works (pdf).
Assured has been involved in the development of the NTS standard as well as the development of the first high capacity, hardware implementation of NTS. NTS will become an important protocol for time synchronization in enterprise and cloud as well as ICS and automotive applications.