std::chrono::utc_clock::from_sys (3) - Linux Man Pages
template <class Duration>
static std::chrono::utc_time<std::common_type_t<Duration, std::chrono::seconds>> (since C++20)
from_sys(const std::chrono::sys_time<Duration>& t);
Converts a sys_time t to a utc_time u that represents the same point in time.
u.time_since_epoch() - t.time_since_epoch() is equal to the number of leap seconds that was inserted between t and 1 January 1970. If t is the exact date of a leap second insertion, that leap second is counted as inserted.
A utc_time representing the same point in time as t.
to_sys converts utc_time to sys_time
clock_cast convert time points of one clock to another