cpp/chrono/month/operator arith 2

@1-2@ Adds months to. The month value held in the result is computed by first evaluating, reducing it modulo 12 to an integer in the range [0, 11], and then adding 1. @3@ Subtracts months from  and returns the result. Equivalent to @4@ If and  are both, returns a std value  such that  is in the range [0, 11] and. Otherwise the returned value is unspecified.

Return value
@1-3@ A std holding a month value calculated as described above. @4@ A std representing the distance between and.