It seems that this is incorrect:

julia> one(Dates.Second)
1 second

The documentation of one says "Get the multiplicative identity element for the type of x". So the correct return would be Int64(1), since then one(Dates.Second) * Dates.Second(5) == Dates.Second(5) as expected. The current behaviour results in a MethodError which is not desirable.

If I did not miss something obvious, I could make a pull request.