Proposed resolution:

This wording is relative to N4910.

1. Modify [time.syn], header <chrono> synopsis, as indicated:

   […]
   // [time.duration.alg], specialized algorithms
   template<class Rep, class Period>
     constexpr common_type_t<duration<Rep, Period>> abs(duration<Rep, Period> d);
   […]
   

2. Modify [time.duration.alg] as indicated:

   [Drafting note: This will cause abs to reduce the period before returning it, much like the other arithmetic operators.

   This is not a breaking change, because code that was using abs with a non-reduced period before did not compile. ]

   template<class Rep, class Period>
     constexpr common_type_t<duration<Rep, Period>> abs(duration<Rep, Period> d);
   

   -1- Constraints: numeric_limits<Rep>::is_signed is true.

   -2- Returns: If d >= d.zero(), return +d, otherwise return -d.