How do I get the current UTC offset (time zone)?
How do I get the current UTC offset (as in time zone, but just the UTC offset of the current moment)?
I need开发者_如何学Python an answer like "+02:00".
There are two parts to this question:
- Get the UTC offset as a
boost::posix_time::time_duration
- Format the
time_duration
as specified
Apparently, getting the local time zone is not exposed very well in a widely implemented API. We can, however, get it by taking the difference of a moment relative to UTC and the same moment relative to the current time zone, like this:
boost::posix_time::time_duration get_utc_offset() {
using namespace boost::posix_time;
// boost::date_time::c_local_adjustor uses the C-API to adjust a
// moment given in utc to the same moment in the local time zone.
typedef boost::date_time::c_local_adjustor<ptime> local_adj;
const ptime utc_now = second_clock::universal_time();
const ptime now = local_adj::utc_to_local(utc_now);
return now - utc_now;
}
Formatting the offset as specified is just a matter of imbuing the right time_facet
:
std::string get_utc_offset_string() {
std::stringstream out;
using namespace boost::posix_time;
time_facet* tf = new time_facet();
tf->time_duration_format("%+%H:%M");
out.imbue(std::locale(out.getloc(), tf));
out << get_utc_offset();
return out.str();
}
Now, get_utc_offset_string()
will yield the desired result.
Since C++11 you can use chrono and std::put_time:
#include <chrono>
#include <iomanip>
#include <iostream>
int main ()
{
using sc = std::chrono::system_clock;
auto tm = sc::to_time_t(sc::now());
std::cout << std::put_time(std::localtime(&tm), "formatted time: %Y-%m-%dT%X%z\n");
std::cout << "just the offset: " << std::put_time(std::localtime(&tm), "%z\n");
}
This produces the following output:
formatted time: 2018-02-15T10:25:27+0100
just the offset: +0100
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