How do I convert "2012-03-02" into unix epoch time in C?

c datetime date time

15,303

Solution 1

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().

Solution 2

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.

Solution 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);
}

15,303

Author by

user1068636

Updated on June 12, 2022

Comments

user1068636 almost 2 years

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?
Matteo Italia about 12 years

That's not standard, C, it's POSIX.
rnrneverdies almost 5 years

tm_year is current year - 1900.
Sopalajo de Arrierez over 4 years

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.
R.. GitHub STOP HELPING ICE over 4 years

@SopalajodeArrierez: If you have tm_gmtoff you almost surely have a POSIX superset and then you have better more portable solutions via POSIX.
Gow. almost 3 years

Is there any other formula/source where instead of tm_yday, I can use tm_mon and tm_mday?
R.. GitHub STOP HELPING ICE almost 3 years

@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.
Gow. almost 3 years

stackoverflow.com/questions/67532995/…
Gow. over 2 years

Can you tell why we need to subtract 1900 and 1 from tm_year and tm_mon ?
Matteo Italia over 2 years

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.