newctime(3)

📦 version 2026f, in arch rolling, nixpkgs unstable

✨ newctime(3) — convert date and time ✨

okay bestie, gather round, because this is the Library Functions Manual entry for newctime and it's literally the entire clock-and-calendar friend group. six functions, one header, and a whole lot of tm. let's get into it 💖

🎀 NAME 🎀

asctime, ctime, difftime, gmtime, localtime, mktime — convert date and time

🌸 SYNOPSIS 🌸

#include <time.h>

struct tm *localtime(time_t const *clock);
struct tm *localtime_r(time_t const *restrict clock,
    struct tm *restrict result);
struct tm *localtime_rz(timezone_t restrict zone,
    time_t const *restrict clock,
    struct tm *restrict result);

struct tm *gmtime(time_t const *clock);
struct tm *gmtime_r(time_t const *restrict clock,
    struct tm *restrict result);

time_t mktime(struct tm *tm);
time_t mktime_z(timezone_t restrict zone,
    struct tm *restrict tm);

double difftime(time_t time1, time_t time0);

[[deprecated]] char *asctime(struct tm const *tm);
[[deprecated]] char *ctime(time_t const *clock);

/* Only in POSIX.1-2017 and earlier.  */
char *ctime_r(time_t const *clock, char *buf);
char *asctime_r(struct tm const *restrict tm,
    char *restrict result);

cc ... -ltz

yes you do have to link it. cc ... -ltz, that's the vibe. 🫶

💅 DESCRIPTION 💅

okay so first up: localtime and gmtime take an integer, the one pointed to by clock, and hand you back pointers to “tm” structures (the full rundown is below, don't skip it, it's the main character). if that integer is out of range for conversion, they just return a null pointer — no drama, no error message, just null. 😭

localtime corrects for the time zone and any time zone adjustments (like Daylight Saving Time in the United States). gmtime converts to Coordinated Universal Time.

The *clock timestamp is the time in seconds since 1970-01-01 00:00:00 Coordinated Universal Time (UTC). POSIX says timestamps must be nonnegative and must ignore leap seconds. many implementations extend POSIX by allowing negative timestamps, and can therefore represent timestamps that predate the introduction of UTC and are some other flavor of Universal Time (UT). and some implementations support leap seconds, in contradiction to POSIX. messy? yes. iconic? also yes. 💀

The mktime function converts the broken-down time, expressed as local time, in the structure pointed to by tm into a calendar time value with the same encoding as that of the values returned by the time function. The original values of the tm_wday and tm_yday components of the structure are ignored, and the original values of the other components are not restricted to their normal ranges. (a positive or zero value for tm_isdst causes mktime to presume initially that daylight saving time respectively, is or is not in effect for the specified time. a negative value for tm_isdst causes mktime to attempt to divine whether daylight saving time is in effect for the specified time; in this case it does not use a consistent rule and may give a different answer when later presented with the same argument. 🔮)

On successful completion, the values of the tm_wday and tm_yday components of the structure are set appropriately, and the other components are set to represent the specified calendar time, but with their values forced to their normal ranges; the final value of tm_mday is not set until tm_mon and tm_year are determined. mktime returns the specified calendar time. If the calendar time cannot be represented, it returns -1 without updating the structure.

🚨 and here's the trick to tell failure apart from a totally valid -1 return: set tm_wday or tm_yday to a negative value before calling mktime. if that value is still negative when mktime returns, the calendar time could not be represented. iykyk. ✨

The difftime function returns the difference between two calendar times, (time1 - time0), expressed in seconds. that's it. that's the whole function. queen behavior. 👑

🦋 the string ones (asctime & ctime) 🦋

The asctime function converts a time value contained in a “tm” structure to a pointer to a string of the form

Thu Nov 24 18:22:48 1986\n\0

Years requiring fewer than four characters are padded with leading zeroes. For years longer than four characters, the string is of the form

Thu Nov 24 18:22:48     81986\n\0

with five spaces before the year. This unusual format is designed to make it less likely that older software that expects exactly 26 bytes of output will mistakenly output misleading values for out-of-range years. not me defending the ancients, but okay, thoughtful. 💅

This function is deprecated starting in C23. Callers can use strftime instead.

The ctime function is equivalent to calling localtime and then calling asctime on the result. Like asctime, this function is deprecated starting in C23. Callers can use localtime and strftime instead.

🌈 the _r ones (reentrant girlies) 🌈

The ctime_r, localtime_r, gmtime_r, and asctime_r functions are like their unsuffixed counterparts, except that they accept an additional argument specifying where to store the result if successful.

The ctime_r and asctime_r functions are present only on systems supporting POSIX.1-2017 and earlier, as they are removed in POSIX.1-2024 and user code can define these functions with other meanings. so like, they're on the way out bestie, don't get too attached. 😭

🧚 the _z ones (zone girlies) 🧚

The localtime_rz and mktime_z functions are like their unsuffixed counterparts, except that they accept an extra initial zone argument specifying the timezone to be used for conversion.

if zone is null, UT is used
otherwise, zone should be have been allocated by tzalloc and should not be freed until after all uses (e.g., by calls to strftime) of the filled-in tm_zone fields

💖 the struct tm reveal 💖

Declarations of all the functions and externals, and the “tm” structure, are in the <time.h> header file. The structure (of type) struct tm includes the following fields:

int tm_sec;      /* seconds (0–60) */
int tm_min;      /* minutes (0–59) */
int tm_hour;     /* hours (0–23) */
int tm_mday;     /* day of month (1–31) */
int tm_mon;      /* month of year (0–11) */
int tm_year;     /* year - 1900 */
int tm_wday;     /* day of week (Sunday = 0) */
int tm_yday;     /* day of year (0–365) */
int tm_isdst;    /* is daylight saving time in effect? */
char *tm_zone;   /* time zone abbreviation (optional) */
long tm_gmtoff;  /* offset from UT in seconds (optional) */

The tm_isdst field is non-zero if daylight saving time is in effect.

The tm_gmtoff field is the offset (in seconds) of the time represented from UT, with positive values indicating east of the Prime Meridian. The field's name is derived from Greenwich Mean Time, a precursor of UT. fun little etymology moment, we love to see it. 📚

In platforms conforming to POSIX.1-2024 the struct tm the tm_zone and tm_gmtoff fields exist, and are filled in. For localtime_rz and mktime_rz the storage lifetime of the strings addressed by tm_zone extends until the corresponding timezone_t object is freed via tzfree. For the other functions the lifetime extends until the TZ environment variable changes state and tzset is then called. ⏳

As a side effect, the ctime, localtime and mktime functions also behave as if tzset were called. The ctime_r and localtime_r functions might (or might not) also behave this way. This is for compatibility with older platforms, as required by POSIX. 🙈

🍓 FILES 🍓

/etc/localtime — local timezone file
/usr/share/zoneinfo — timezone directory
/usr/share/zoneinfo/GMT — for UTC leap seconds

If /usr/share/zoneinfo/GMT is absent, UTC leap seconds are loaded from /usr/share/zoneinfo/GMT0 if present.

🌟 SEE ALSO 🌟

getenv(3), newstrftime(3), newtzset(3), time(2), tzfile(5).

👑 NOTES 👑

The return values of asctime, ctime, gmtime, and localtime point to static data overwritten by each call. the remaining functions and data are thread-safe. 🫶

⚠️ so if you're holding onto one of those pointers and calling again, you're holding onto a ghost. it's giving shared buffer chaos.

The asctime, asctime_r, ctime, and ctime_r functions behave strangely for years before 1000 or after 9999. The 1989 and 1999 editions of the C Standard say that years from -99 through 999 are converted without extra spaces, but this conflicts with longstanding tradition and with this implementation. The 2011 edition says that the behavior is undefined if the year is before 1000 or after 9999. Traditional implementations of these two functions are restricted to years in the range 1900 through 2099. To avoid this portability mess, new programs should use strftime instead.

🚨 real talk: undefined behavior is not a vibe. if your program is doing year math with these, please, for me, switch to strftime.

and that's the whole page, bestie. this one's from the <strong>Time Zone Database</strong> crew, page <code>newctime(3)</code> — so next time someone asks you what time it is, you can say "well, actually, it depends on the timezone_t, the TZ environment variable, and whether the calendar time could be represented at all." 💖✨ slay, periodt.

📦 where this page comes from

arch rolling: tzdata 2026e-1
arch rolling: tzdata 2026f-1 · LicenseRef-tz
nixpkgs unstable: tzdata 2026e
💖 girlified by deepseek-ai/DeepSeek-V4.1-Flash on 2026-10-14: a language model's rewrite of the original, under its terms