A string "2012-03-02" representing March 2nd, 2012 is given to me as an input variable (char *).
How do I convert this date into unix epoch time in C programming language?
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5 Answers
4
Local time or UTC? If it's UTC, the easiest way to do the conversion is to avoid the C time API entirely and use the formula in POSIX for seconds since the epoch:
tm_sec + tm_min*60 + tm_hour*3600 + tm_yday*86400 +
(tm_year-70)*31536000 + ((tm_year-69)/4)*86400 -
((tm_year-1)/100)*86400 + ((tm_year+299)/400)*86400
Source: http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap04.html#tag_04_15
If it's local time, the problem turns into hell due to the fact that time_t is not guaranteed to be represented as seconds since the epoch except on POSIX systems, and the fact that it's difficult to compute a time_t value corresponding to the epoch (mktime will not work because it uses local time). Once you compute the time_t for the epoch, though, it's just a matter of using mktime for the time value you parsed and then calling difftime.
R.. GitHub STOP HELPING ICE
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`tm_year` is current year - 1900. – rnrneverdies May 27 '19 at 03:59
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What about doing `results=results-tm_gmtoff` to readjust by using the time zone offset? I think it is not a completely standard C (rather a extension), but I have tested it right now and it seems to be working OK. – Sopalajo de Arrierez Jan 18 '20 at 16:29
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@SopalajodeArrierez: If you have tm_gmtoff you almost surely have a POSIX superset and then you have better more portable solutions via POSIX. – R.. GitHub STOP HELPING ICE Jan 18 '20 at 21:09
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Is there any other formula/source where instead of tm_yday, I can use tm_mon and tm_mday? – Gow. May 13 '21 at 08:30
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@Gow: Yes, it requires some arithmetic with number of days in each month though. Perhaps open it as a new question if you want details, referencing this question. – R.. GitHub STOP HELPING ICE May 13 '21 at 12:20
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https://stackoverflow.com/questions/67532995/how-to-convert-date-into-epoch-timestamp-using-the-tm-mday-and-tm-mon-in-formula – Gow. May 14 '21 at 10:42
3
Extract the pieces with an sscanf, populate struct tm (from <time.h>) with the data extracted, and finally use mktime to convert it to time_t.
time_t ParseDate(const char * str)
{
struct tm ti={0};
if(sscanf(str, "%d-%d-%d", &ti.tm_year, &ti.tm_mon, &ti.tm_day)!=3)
{
/* ... error parsing ... */
}
ti.tm_year-=1900
ti.tm_mon-=1
return mktime(&ti);
}
Matteo Italia
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Can you tell why we need to subtract 1900 and 1 from tm_year and tm_mon ? – Gow. Aug 28 '21 at 09:29
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Because in `struct tm` years start with 1900, and the month is 0-based. You find all the relevant information about this in the documentation. – Matteo Italia Aug 28 '21 at 18:55
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C (POSIX) provides a function for this. Use strptime() to convert the string into a struct tm value. You can then convert the struct tm into time_t using mktime().
Jonathan Leffler
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Aditya Naidu
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0
Here is my solution:
#include <stdio.h>
#include <string.h>
#include <time.h>
int main(void) {
time_t epoch;
char timeString[80] = { "05 07 2021 00 33 51" };
struct tm my_tm;
char buffer[80];
memset(&my_tm, 0, sizeof(my_tm));
if (sscanf(timeString, "%d %d %d %d %d %d", &my_tm.tm_mon, &my_tm.tm_mday, &my_tm.tm_year, &my_tm.tm_hour, &my_tm.tm_min, &my_tm.tm_sec) != 6)
{
/* ... error parsing ... */
printf(" sscanf failed");
return 1;
}
my_tm.tm_isdst = -1;
my_tm.tm_year -= 1900;
my_tm.tm_mon -= 1;
epoch = mktime(&my_tm);
if (epoch == -1) {
printf("Error: unable to make time using mktime\n");
}
else {
strftime(buffer, sizeof(buffer), "%c", &my_tm);
printf("%s (epoch=%ld)", buffer, (long)epoch);
}
return(0);
}
Note: This code convert the input you provided to "Fry May 7 00:33:51" and epoch=1620340431 which is not the one you mention in comment (Running under Win10 with MSVC)
fpiette
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