如何在UNIX上使用C日期和时间函数？

Question

Jon Skeet spoke of the complexity of programming dates and times at the 2009 DevDays in London.

Can you give me an introduction to the ANSI C date/time functions on UNIX and indicate some of the deeper issues I should also consider when using dates and times?

Answer 1

Terminology

A date/time can be in two formats:

• calendar time (aka simpletime) – time as an absolute value typically since some base time, often referred to as the Coordinated Universal Time
• localtime (aka broken-down time) – a calendar time made up of components of year, month, day etc. which takes into account the local time zone including Daylight Saving Time if applicable.

Data types

The date/time functions and types are declared in the time.h header file.

Time can be stored as a whole number or as an instance of a structure:

• as a number using the time_t arithmetic type – to store calendar time as the number of seconds elapsed since the UNIX epoch January 1, 1970 00:00:00

• using the structure timeval – to store calendar time as the number of seconds and nanoseconds elapsed since the UNIX epoch January 1, 1970 00:00:00

• using the structure tm to store localtime, it contains attributes such as the following:

   tm_hour tm_min tm_isdst 

The tm_isdst attribute above is used to indicate Daylight Saving Time (DST). If the value is positive it is DST, if the value is 0 it is not DST.

Program to print the current Coordinated Universal Time

#include <stdio.h>
#include <time.h>

int main ( int argc, char *argv[] )
{
    time_t now;

    now = time ( NULL );

    printf ( "It’s %ld seconds since January 1, 1970 00:00:00", (long) now );

    return 0;
}

In the program above the function time reads the UNIX system time, subtracts that from January 1, 1970 00:00:00 (the UNIX epoch) and returns its result in seconds.

Program to print the current local time

#include <stdio.h>
#include <time.h>

int main ( int argc, char *argv[] )
{
    time_t now;
    struct tm *lcltime;

    now = time ( NULL );
    lcltime = localtime ( &now );

    printf ( "The time is %d:%d\n", lcltime->tm_hour, lcltime->tm_min );

    return 0;
}

In the program above the function localtime converts the elapsed time in seconds from the UNIX epoch into the broken-down time. localtime reads the UNIX environment TZ (through a call to the tzset function) to return the time relative to the timezone and to set the tm_isdst attribute.

A typical setting of the TZ variable in UNIX (using bash) would be as follows:

export TZ=GMT

or

export TZ=US/Eastern

Program to print the current formatted Greenwich Mean Time

#include <stdio.h>
#include <time.h>

int main ( int argc, char *argv[] )
{
    time_t now;
    struct tm *gmt;
    char formatted_gmt [50];

    now = time ( NULL );
    gmt = gmtime ( &now );

    strftime ( formatted_gmt, sizeof(formatted_gmt), "%I:%M %p", gmt );
    printf ( "The time is %s\n", formatted_gmt );

    return 0;
}

In the program above the function strftime provides specialised formatting of dates.

Other issues to consider

• Leap seconds
• What should we do to prepare for 2038?