Is there any simple way to get daylight saving time transition time under Linux in C/C++

Question

I want to get the transition time for DST Under Linux with giving time zone or TZ env. My way is stupid, giving the start of the year and try every hour then check tm_isdst value of local time to get the transition time. Is there some simple way to do this?

Answer 1

There is the source code in glibc, which you can browse here: http://sourceware.org/git/?p=glibc.git;a=tree;f=timezone

Or you can use the timezone database here: ftp://ftp.iana.org/tz/releases/tzdata2012c.tar.gz

Since you haven't given a particular timezone/location, I can't look up and give you the exact information for you.

Answer 2

You can also use boost_datetime.

#include <iostream>
#include <boost/date_time/local_time/local_time.hpp>

using namespace boost::local_time;
using namespace boost::posix_time;

int main()
{
  tz_database tz_db;
  tz_db.load_from_file("/path_to_boost/boost/libs/date_time/data/date_time_zonespec.csv");
  time_zone_ptr zone = tz_db.time_zone_from_region("America/New_York");
  ptime t1 = zone->dst_local_start_time(2013);
  ptime t2 = zone->dst_local_end_time(2013);
  std::cout << t1 << std::endl;
  std::cout << t2 << std::endl;
}

Some related SO links: c++ How to find the time in foreign country taking into account daylight saving? How do you get the timezone (offset) for a location on a particular date?

But as RedX said earlier, politics may change time zones. So actually your original solution has the advantage of being automatically updated with the underlying OS. Also, you can improve your existing solution by using binary search.