std::chrono::milliseconds is a C++ duration type representing intervals in one-millisecond units. It belongs to the std::chrono family alongside minutes, hours, days, and other durations. section 4.2.2
Conversions between durations can be implicit when moving from a coarser interval to a finer one. Conversions that may lose precision require duration_cast. section 4.2.2
Convert two hours to milliseconds
std::chrono::milliseconds ms = std::chrono::hours(2); This assignment converts a coarser two-hour duration into a finer-grained millisecond duration. section 4.2.2
auto two_hours_from_ms = duration_cast<hours>(ms);
Use duration_cast when converting milliseconds back to hours, because the conversion can discard fractional precision. section 4.2.2
