Datetime parse and format in C++

Question

I'm using time_point the first time. I want to parse datetime from string and found a difference when returning back to string from 1 hour.

std::chrono::system_clock::time_point timePoint;
std::stringstream ss("2021-01-01 00:00:09+01");
std::chrono::from_stream(ss, "%F %T%z", timePoint);  
  // timePoint == {_MyDur={_MyRep=16094556090000000 } }
std::string timePointStr = std::format("{:%Y/%m/%d %T}", floor<std::chrono::seconds>(timePoint));
  // timePointStr = "2020/12/31 23:00:09"

I don't know what is wrong: the timepoint and the parsing or the formatting to string? How can I get the same format as the parsed one?

Answer 1

This is the expected behavior.

Explanation:

system_clock::time_point, and more generally, all time_points based on system_clock, have the semantics of Unix Time. This is a count of time since 1970-01-01 00:00:00 UTC, excluding leap seconds.

So when "2021-01-01 00:00:09+01" is parsed into a system_clock::time_point, the "2021-01-01 00:00:09" is interpreted as local time, and the "+01" is used to transform that local time into UTC. When formatting back out, there is no corresponding transformation back to local time (though that is possible with additional syntax¹). The format statement simply prints out the UTC time (an hour earlier).

If you would prefer to parse "2021-01-01 00:00:09+01" without the transformation to UTC, that can be done by parsing into a std::chrono::local_time of whatever precision you desire. For example:

std::chrono::local_seconds timePoint;
std::stringstream ss("2021-01-01 00:00:09+01");
from_stream(ss, "%F %T%z", timePoint);
...

Now when you print it back out, you will get "2021/01/01 00:00:09". However the value in the rep is now 1609459209 (3600 seconds later).

---

¹ To format out at sys_time as a local_time with a UTC offset of 1h it is necessary to choose a time_zone with a UTC offset of 1h at least at the UTC time you are formatting. For example the IANA time zone of "Etc/GMT-1" always has an offset of 1h ... yes, the signs of the offset are reversed. Using this to transform 2020-12-31 23:00:09 UTC back to 2021-01-01 00:00:09 would look like:

std::chrono::sys_seconds timePoint;
std::stringstream ss("2021-01-01 00:00:09+01");
std::chrono::from_stream(ss, "%F %T%z", timePoint);
// timePoint == 2020-12-31 23:00:09 UTC
std::string timePointStr = std::format("{:%Y/%m/%d %T}",
                               std::chrono::zoned_time{"Etc/GMT-1", timePoint});
cout << timePointStr << '\n';  // 2021/01/01 00:00:09

Disclaimer: I do not currently have a way to verify that MSVC supports the "Etc/GMT-1" std::chrono::time_zone.

Fwiw, using "Africa/Algiers" or "Europe/Amsterdam" in place of "Etc/GMT-1" should give the same result for this specific time stamp. And if your computer has its local time zone set to something that has a 1h UTC offset for this timestamp, then std::chrono::current_zone() in place of of "Etc/GMT-1" will also give the same result.