add seconds to a date

Question

I need to add seconds to a date. For example, if I have a date such as 2009127000000, I need to add the seconds to this date. Another example, add 50 seconds to 200912312359

Answer 1

The C date/time type time_t is implemented as the number of seconds since a certain date, so to add seconds to it you simply use normal arithmetic. If this is not what you are asking about please make your question clearer.

Answer 2

Use types and functions from <time.h>.

time_t now = time(0);
time_t now_plus_50_seconds = now + 50;
time_t now_plus_2_hours = now + 7200;

<time.h> declares functions that deal with time_t and struct tm types. These functions can do all you want.

Answer 3

In POSIX a time_t value is specified to be seconds, however that's not guaranteed by the C standard, so it might not be true on non-POSIX systems. It commonly is (in fact, I'm not sure how often it isn't a value representing seconds).

Here's an example of adding time values that doesn't assume a time_t represents seconds using the standard library facilities, which are really not particularly great for manipulating time:

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

int main()
{
    time_t now = time( NULL);

    struct tm now_tm = *localtime( &now);


    struct tm then_tm = now_tm;
    then_tm.tm_sec += 50;   // add 50 seconds to the time

    mktime( &then_tm);      // normalize it

    printf( "%s\n", asctime( &now_tm));
    printf( "%s\n", asctime( &then_tm));

    return 0;
}

Parsing your time string into an appropriate struct tm variable is left as an exercise. The strftime() function can be used to format a new one (and the POSIX strptime() function can help with the parsing).

Answer 4

Try something like this: (Note: no error checking)

include <time.h>

char* string = ...;
char  buf[80];
struct tm;
strptime(string, "%Y%m...", &tm);
tm->tm_isdst = 0;
strftime(buf, sizeof(buf), "%Y%m...", localtime(mktime(&tm) + 50));