Find timezone difference in the past

[bambuk6]

How do I find the difference (say, in minutes) between some timezone and UTC in the past? Will the library respect the timezone rules that were active at that date in the past?

[HowardHinnant]

It depends on the database, but generally yes. Linux typically installs the full IANA database which has rules dating back to the mid 1800's.

[HowardHinnant]

#include "date/tz.h"
#include <iostream>

int
main()
{
    using namespace std;
    using namespace std::chrono;
    using namespace date;

    auto time = sys_days{1834_y/February/27};
    auto la = zoned_time{"America/Los_Angeles", time};
    auto utc = zoned_time{"Etc/UTC", time};
    cout << format("%T\n", la.get_local_time() - utc.get_local_time());
}

Output:

-07:52:58

For pre-std times, the IANA database typically uses "mean solar time" for the city named by the time zone.