convert string to time_t and then convert time_t back to string

Question

I want to save a date time string to time_t and then convert it back to exactly original string.

But the code below will output "2016-04-25_10:10:05"

And the hour in the output will be incorrect by changing the date_str.

If you change the code to std::string date_str = "1470-04-25_09:10:05";, the result will correct.

Here is the code:

#include <iostream>
#include <ctime>
#include <string>
#include <sstream>
#include <iomanip>

int main()
{
    // try changing year, hour will be incorrect
    std::string date_str = "2016-04-25_09:10:05";

    std::tm tm{};
    std::istringstream str_stream(date_str);
    str_stream >> std::get_time(&tm, "%Y-%m-%d_%T");
    std::time_t time = std::mktime(&tm);

    std::stringstream stream;
    stream << std::put_time(std::localtime(&time), "%F_%T");
    std::cout << stream.str() << std::endl;
}

Time zone problems? What time zone are you in? Daylight saving? Have you checked the `tm` structure for details (in e.g. a debugger)? — Some programmer dude, Aug 05 '16 at 02:33
The 1 hour gap makes sense... what I'm not getting is the year 1470. — pah, Aug 05 '16 at 02:36
@JoachimPileborg It is on the same machine. But how to make it time zone independent. — Christophe, Aug 05 '16 at 02:36
@Christophe *probably* because `tm_isdst` is not set. But the year value is puzzling me... — pah, Aug 05 '16 at 02:40
@SinanÜnür `gcc version 5.3.0 20151204 (Ubuntu 5.3.0-3ubuntu1~14.04)` — Christophe, Aug 05 '16 at 03:51

pah · Accepted Answer · 2016-08-05T12:04:29.853

Daylight Saving Time (DST) is used to save energy and make better use of daylight. It was first used in 1908 in Thunder Bay, Canada.

This explains why any year that you pass prior to 1908 (or prior to the year your timezone adopted DST) will affect the hour.

Also, answering to the one hour gap on the "2016-04-25_10:10:05" case, this is because you're not setting tm.tm_isdst prior to mktime() call:

/* Assuming that all tm memory is set to 0 prior to this */
tm.tm_isdst = -1; /* mktime() will figure out the DST */
std::time_t time = std::mktime(&tm);

According to POSIX-1003.1-2001:

A positive or 0 value for tm_isdst shall cause mktime() to presume initially that Daylight Savings Time, respectively, is or is not in effect for the specified time. A negative value for tm_isdst shall cause mktime() to attempt to determine whether Daylight Savings Time is in effect for the specified time.

according to [this answer](http://stackoverflow.com/a/12115059/238902) `-1` is interpreted as `unknown` and might be unpredictable. Shouldn't it be `1` instead? — default, Aug 05 '16 at 11:17
I couldn't find anything else about -1 being unpredictable. The qoute seems to be that users personal opinion. Good job! — default, Aug 05 '16 at 11:31

convert string to time_t and then convert time_t back to string

1 Answers1