Constructing std::chrono::system_clock::time_point from a duration

I have a data source that provides time since the start of the day (00:00:00Z). I want to construct a std::chrono::time_point based in this input.

For the sake of example, the current time is 2020-02-27T01:05:30.687073184. My source provides me a binary-coded decimal value, to 0.01 seconds. I construct intermeditate values to separate the parsing/mediation from the combination.

        using Days = std::chrono::duration<int, std::ratio<86400>>;

        auto hr10  = std::chrono::duration<unsigned, std::ratio<36000>> ( 0 );
        auto hr1   = std::chrono::duration<unsigned, std::ratio<3600>>  ( 1 );
        auto mn10  = std::chrono::duration<unsigned, std::ratio<600>>   ( 0 );
        auto mn1   = std::chrono::duration<unsigned, std::ratio<60>>    ( 5 );
        auto sc10  = std::chrono::duration<unsigned, std::ratio<10>>    ( 3 );
        auto sc1   = std::chrono::duration<unsigned, std::ratio<1>>     ( 0 );
        auto ms100 = std::chrono::duration<unsigned, std::ratio<1, 10>> ( 6 );
        auto ms10  = std::chrono::duration<unsigned, std::ratio<1, 100>>( 8 );

        auto t = hr10 + hr1 + mn10 + mn1 + sc10 + sc1 + ms100 + ms10;  // 393068
        auto now = std::chrono::system_clock::now();
        auto sinceEpoch = now.time_since_epoch();      // 1582765530687073184 ns

        auto today = std::chrono::duration_cast<Days>(sinceEpoch);    // 18319 days
        auto sinceDay = sinceEpoch - today;            // 3930687073184 ns

// There is logic to determine if there is a day roll-over; it is not relevant here
        auto adjust = Days(0);

// Create the time_point
        auto tp = std::chrono::system_clock::time_point();
        std::cout << "\n\tnull:       " << tp.time_since_epoch().count();

        tp += today;
        std::cout << "\n\ttoday:      " << tp.time_since_epoch().count();

        tp += adjust;
        std::cout << "\n\tadjust:     " << tp.time_since_epoch().count();

        tp += t;
        std::cout << "\n\tt:          " << tp.time_since_epoch().count();

        std::cout << "\n\tall-in-one: " << decltype(tp)(today+adjust+t).time_since_epoch().count();

This results in the following output:

        null:       0
        today:      1582761600000000000
        adjust:     1582761600000000000
        t:          1582765530680000000
        all-in-one: 36577304120000000

What I don't understand is why incrementally adding each duration to the time_point produces the desired effect, but trying to construct the time_point from an aggregate duration does not.