Trying to add 1 month to today does not yield a sys_days!!! [...] next_month's type is time_point<system_clock, duration<int, ratio<54>> 
duration<int, ratio<54>> is the type used to represent the sum of n
days and m
months.
A
duration is a count of ticks in units of
p/q seconds. For
std::chrono::days, the unit
p/q is 86400 seconds. 86400 seconds = 1 day. For
months it's 2629746 seconds = 1 month.
Those numbers come from the definition of
days and
months:
1 2

using days = duration<int, std::ratio<86400>>;
using months = duration<int, std::ratio<2629746>>;

Using those numbers the relevant sum is:
n day + m month = n * 86400sec + m * 2629746sec
To evaluate it and store the result in a
duration we need to choose just one unit
p/q that is known at compiletime.
To do it, the compiler finds
gcd(86400, 2629746) = 54 at compile time and factors each term:
54n * 1600sec + 54m * 48699sec = (1600n + 48699m) * 54sec
This says the duration that holds the sum should store
(1600n + 48699m) "almostminutes" where each "almostminute" is 54 seconds long.
can't I cast to duration<days>? 
Since these durations are stored in
time_points, use
time_point_cast instead:
time_point_cast<days>(next_month);