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 20091231235957.
Is this possible in C?
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I think the date format in your example is underspecified. 2009127000000 could be the 7th of December or the 27th of January.Thomas– Thomas2009-12-07 10:43:01 +00:00Commented Dec 7, 2009 at 10:43
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@benjamin, have a read of the man pages for ctime (date / time conversion functions) and strptime (converts string representation of a time to a time tm structure)Glen– Glen2009-12-07 10:53:05 +00:00Commented Dec 7, 2009 at 10:53
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4 Answers
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).
5 Comments
Lazer
how do I get back a
time_t from the modified then_tm?
Steve Jessop
@Lazer:
mktime returns the time_t you want.
abhi
@Michael Burr. This will be true only if seconds is less then 60. I mean otherwise you have to do the entire calculation yourself
cesargastonec
@abhi why? it works with any seconds you want: 50, 200, 3600, 86400 etc. Obviously the user will know how much time is 3600 seconds.
Vassilis
No guys, the seconds and minutes you have to manage yourself. Adding 120 to
tm_min gives something like Current time: 19:56:21 will fire at: 19:176:21 using std::put_time(my_tm,"%X")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.
4 Comments
MyDaftQuestions
Should be the answer. Clean, clear, and has excellent examples. +1
pmg
Updated
<time.h> from POSIX 2016: pubs.opengroup.org/onlinepubs/9699919799/basedefs/time.h.html
Meric Ozcan
This is the universal answer other does not properly solves
TimeS
This does not work if time_t is not expressed in whole seconds (not guaranteed on all platforms).
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.
2 Comments
Michael Burr
time_t is commonly seconds, but isn't necessarily so.
Han
Beware that this answer is not correct all the time. Please see the @MichaelBurr's answer: stackoverflow.com/a/1860996/1776916
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));
