#include <chrono> int main() { using clock = std::chrono::system_clock; using time_point = std::chrono::time_point<clock>; auto tp_now = clock::now(); auto tp_min = time_point::min(); bool b1 = tp_now > tp_min; bool b2 = (tp_now - tp_min) > std::chrono::seconds{ 0 }; cout << boolalpha << b1 << endl << b2 << endl; } The expected output is:
true
true
But the actual output is:
true
false
Why does std::chrono::time_point not behave as expected?
With:
using clock = std::chrono::system_clock; using time_point = std::chrono::time_point<clock>; time_point is implemented as if it stores a value of type Duration indicating the time interval from the start of the Clock's epoch. (See std::chrono::time_point)
The duration member type of clock (and of time_point) is capable of representing negative durations.
Thus duration in your implementation may be implemented with a back-end signed integer, (it can be implemented with unsigned integer but with a complicated comparison).
In that particular implementation,
time_point::min(); time_point t(clock::duration::min()); time_point t(clock::duration(std::numeric_limits<Rep>::lowest())); and tp_now is greater than zero, thus when you subtract them, you get an integer overflow because the result is larger than std::numeric_limits<Rep>::max(). In implementation with signed back-end, it's undefined behavior, in implementation with unsigned back-end, I don't know about it, but I guess its special comparison will make its false.
In this example, tp_min is -9223372036854775808 ticks from its epoch, that number is the same with std::numeric_limits<duration::rep>::lowest()
TL;DR; It's integer overflow. Don't use
(tp1 - tp2) > std::chrono::duration<whatever_rep>::zero Instead, use
tp1 > tp2 
tp_now - tp_minis negative on Wandbox. I think Danh is right.