For the record:
I have published the first version of AstroTime.jl.
We had a discussion about the accuracy and resolution here: https://github.com/JuliaAstro/AstroTime.jl/issues/28
In theory we should be able to achieve an accuracy of 55 attoseconds for a range of 10x the age of the universe (I have not tested these extremes though because most date-based algorithms become meaningless there). Shifting an epoch with a subsecond offset close to machine precision by a million years works though (see here) 