Self nomination of hifitime, currently in version 3.1 (and no-std for most).
It's a high fidelity time management library that guarantees exactly one nanosecond precision for 32,768 years before 01 January 1900 and 32,767 years after that 01 January 1900. It supports several time systems: TAI, UTC, ET, TDB, and GPS time. It also includes high precision duration calculations like let dur: Duration = 23.hours() + 14.minutes();, so adding time zone support should straight forward. All epochs are stored in TAI under the hood in just 80 bits!