strftime(3)

✨ strftime(3) — string format date and time ✨

okay bestie, this is the Library Functions Manual page for strftime, and it is SO much, buckle up 💖

---

🌸 NAME 🌸

strftime — string format date and time

---

🎀 LIBRARY 🎀

Standard C library (libc, -lc)

---

✨ SYNOPSIS ✨

#include <time.h>

size_t strftime(size_t max;
                char s[restrict max], size_t max,
                const char *restrict format,
                const struct tm *restrict tm);

size_t strftime_l(size_t max;
                char s[restrict max], size_t max,
                const char *restrict format,
                const struct tm *restrict tm,
                locale_t locale);

---

💅 DESCRIPTION 💅

so the strftime() function takes your broken-down time tm and formats it according to the format specification format, then drops the result into the character array s which has size max. The broken-down time struct tm lives in <time.h>. Also go peek at ctime(3) bestie 🫶

The format specification is a null-terminated string and it can contain special character sequences called conversion specifications. Each one starts with a '%' character and ends with some other character known as the conversion specifier character. Everything else that isn't one of those is just an ordinary character sequence.

The characters of ordinary character sequences (including the null byte) get copied verbatim from format to s. But the characters of conversion specifications get replaced like the list below says. And in that list, the field(s) used from the tm structure are shown too. slay ✨

---

• %a — The abbreviated name of the day of the week according to the current locale. (Calculated from tm_wday.) (The specific names used in the current locale can be obtained by calling nl_langinfo(3) with ABDAY_{1–7} as an argument.) 🗓️
• %A — The full name of the day of the week according to the current locale. (Calculated from tm_wday.) (The specific names used in the current locale can be obtained by calling nl_langinfo(3) with DAY_{1–7} as an argument.) 👑
• %b — The abbreviated month name according to the current locale. (Calculated from tm_mon.) (The specific names used in the current locale can be obtained by calling nl_langinfo(3) with ABMON_{1–12} as an argument.)
• %B — The full month name according to the current locale. (Calculated from tm_mon.) (The specific names used in the current locale can be obtained by calling nl_langinfo(3) with MON_{1–12} as an argument.)
• %c — The preferred date and time representation for the current locale. (The specific format used in the current locale can be obtained by calling nl_langinfo(3) with D_T_FMT as an argument for the %c conversion specification, and with ERA_D_T_FMT for the %Ec conversion specification.) (In the POSIX locale this is equivalent to %a %b %e %H:%M:%S %Y.)
• %C — The century number (year/100) as a 2-digit integer. (SU) (The %EC conversion specification corresponds to the name of the era.) (Calculated from tm_year.)
• %d — The day of the month as a decimal number (range 01 to 31). (Calculated from tm_mday.)
• %D — Equivalent to %m/%d/%y. (Yecch—for Americans only. Americans should note that in other countries %d/%m/%y is rather common. This means that in international context this format is ambiguous and should not be used.) (SU) 💀
• %e — Like %d, the day of the month as a decimal number, but a leading zero is replaced by a space. (SU) (Calculated from tm_mday.)
• %E — Modifier: use alternative ("era-based") format; see below. (SU)
• %F — Equivalent to %Y-%m-%d (the ISO 8601 date format). (C99)
• %G — The ISO 8601 week-based year (see NOTES) with century as a decimal number. The 4-digit year corresponding to the ISO week number (see %V). This has the same format and value as %Y, except that if the ISO week number belongs to the previous or next year, that year is used instead. (TZ) (Calculated from tm_year, tm_yday, and tm_wday.)
• %g — Like %G, but without century, that is, with a 2-digit year (00–99). (TZ) (Calculated from tm_year, tm_yday, and tm_wday.)
• %h — Equivalent to %b. (SU)
• %H — The hour as a decimal number using a 24-hour clock (range 00 to 23). (Calculated from tm_hour.)
• %I — The hour as a decimal number using a 12-hour clock (range 01 to 12). (Calculated from tm_hour.)
• %j — The day of the year as a decimal number (range 001 to 366). (Calculated from tm_yday.)
• %k — The hour (24-hour clock) as a decimal number (range 0 to 23); single digits are preceded by a blank. (See also %H.) (Calculated from tm_hour.) (TZ)
• %l — The hour (12-hour clock) as a decimal number (range 1 to 12); single digits are preceded by a blank. (See also %I.) (Calculated from tm_hour.) (TZ)
• %m — The month as a decimal number (range 01 to 12). (Calculated from tm_mon.)
• %M — The minute as a decimal number (range 00 to 59). (Calculated from tm_min.)
• %n — A newline character. (SU)
• %O — Modifier: use alternative numeric symbols; see below. (SU)
• %p — Either "AM" or "PM" according to the given time value, or the corresponding strings for the current locale. Noon is treated as "PM" and midnight as "AM". (Calculated from tm_hour.) (The specific string representations used for "AM" and "PM" in the current locale can be obtained by calling nl_langinfo(3) with AM_STR and PM_STR, respectively.)
• %P — Like %p but in lowercase: "am" or "pm" or a corresponding string for the current locale. (Calculated from tm_hour.) (GNU)
• %r — The time in a.m. or p.m. notation. (SU) (The specific format used in the current locale can be obtained by calling nl_langinfo(3) with T_FMT_AMPM as an argument.) (In the POSIX locale this is equivalent to %I:%M:%S %p.)
• %R — The time in 24-hour notation (%H:%M). (SU) For a version including the seconds, see %T below.
• %s — The number of seconds since the Epoch, 1970-01-01 00:00:00 +0000 (UTC). (TZ) (Calculated from mktime(tm).)
• %S — The second as a decimal number (range 00 to 60). (The range is up to 60 to allow for occasional leap seconds.) (Calculated from tm_sec.)
• %t — A tab character. (SU)
• %T — The time in 24-hour notation (%H:%M:%S). (SU)
• %u — The day of the week as a decimal, range 1 to 7, Monday being 1. See also %w. (Calculated from tm_wday.) (SU)
• %U — The week number of the current year as a decimal number, range 00 to 53, starting with the first Sunday as the first day of week 01. See also %V and %W. (Calculated from tm_yday and tm_wday.)
• %V — The ISO 8601 week number (see NOTES) of the current year as a decimal number, range 01 to 53, where week 1 is the first week that has at least 4 days in the new year. See also %U and %W. (Calculated from tm_year, tm_yday, and tm_wday.) (SU)
• %w — The day of the week as a decimal, range 0 to 6, Sunday being 0. See also %u. (Calculated from tm_wday.)
• %W — The week number of the current year as a decimal number, range 00 to 53, starting with the first Monday as the first day of week 01. (Calculated from tm_yday and tm_wday.)
• %x — The preferred date representation for the current locale without the time. (The specific format used in the current locale can be obtained by calling nl_langinfo(3) with D_FMT as an argument for the %x conversion specification, and with ERA_D_FMT for the %Ex conversion specification.) (In the POSIX locale this is equivalent to %m/%d/%y.)
• %X — The preferred time representation for the current locale without the date. (The specific format used in the current locale can be obtained by calling nl_langinfo(3) with T_FMT as an argument for the %X conversion specification, and with ERA_T_FMT for the %EX conversion specification.) (In the POSIX locale this is equivalent to %H:%M:%S.)
• %y — The year as a decimal number without a century (range 00 to 99). (The %Ey conversion specification corresponds to the year since the beginning of the era denoted by the %EC conversion specification.) (Calculated from tm_year)
• %Y — The year as a decimal number including the century. (The %EY conversion specification corresponds to the full alternative year representation.) (Calculated from tm_year)
• %z — The +hhmm or -hhmm numeric timezone (that is, the hour and minute offset from UTC). (SU)
• %Z — The timezone name or abbreviation.
• %+ — The date and time in date(1) format. (TZ) (Not supported in glibc2.)
• %% — A literal '%' character. 💖

---

okay so some conversion specifications can be modified by putting the E or O modifier before the conversion specifier character, which is basically code for "bestie use an alternative format instead". If the alternative format or specification doesn't exist for the current locale, the behavior will be as if the unmodified conversion specification were used. (SU) 🦋

The Single UNIX Specification mentions %Ec, %EC, %Ex, %EX, %Ey, %EY, %Od, %Oe, %OH, %OI, %Om, %OM, %OS, %Ou, %OU, %OV, %Ow, %OW, %Oy — where the O modifier's whole thing is using alternative numeric symbols (say, roman numerals 😭) and the E modifier's whole thing is using a locale-dependent alternative representation. The rules governing date representation with the E modifier can be obtained by supplying ERA as an argument to a nl_langinfo(3). One example of such alternative forms is the Japanese era calendar scheme in the ja_JP glibc locale. 🎀

strftime_l() is equivalent to strftime(), except it uses the specified locale instead of the current locale. The behavior is undefined if locale is invalid or LC_GLOBAL_LOCALE. 💀

---

🌈 RETURN VALUE 🌈

okay so as long as your result string, terminating null byte included, does not exceed max bytes, strftime() returns the number of bytes (not counting the terminating null byte) placed in the array s. But if the length of the result string (terminating null byte included) would exceed max bytes, then strftime() returns 0, and the contents of the array are undefined. 🙈

Also like, important: the return value 0 does not necessarily mean an error bestie. For example, in many locales %p yields an empty string. An empty format string will likewise yield an empty string. it's giving "zero but make it valid" ✨

---

🦋 ENVIRONMENT 🦋

The environment variables TZ and LC_TIME are used.

---

🎀 ATTRIBUTES 🎀

okay so if you're wondering what all those ~terms~ mean, bestie, go peek at attributes(7) — it explains everything.

Here's the tea for strftime() and strftime_l():

• Interface: strftime(), strftime_l()
• Attribute: Thread safety
• Value: MT-Safe env locale

---

✨ STANDARDS ✨

• strftime() — C11, POSIX.1-2008. she's certified, she's official 💅
• strftime_l() — POSIX.1-2008. also certified ✨

---

🦋 HISTORY 🦋

• strftime() — SVr4, C89. an OG, literally predates most of us
• strftime_l() — POSIX.1-2008.

so here's the lore: there are strict inclusions between the different sets of conversions, meaning each family of specs is a subset of the next one up:

• ANSI C conversions (those are the unmarked ones)
• the Single UNIX Specification ones (marked SU)
• Olson's timezone package ones (marked TZ)
• glibc ones (marked GNU)

with one tiny exception: %+ is not supported in glibc2 💀. but glibc2 does have several more extensions, so like, it's fine, she's got range.

also: POSIX.1 only refers to ANSI C, but POSIX.2 describes under date(1) a bunch of extensions that could totally apply to strftime() too. and the %F conversion? that one's in C99 and POSIX.1-2001.

and a lil historical moment: in SUSv2, the %S specifier allowed a range of 00 to 61, to allow for the theoretical possibility of a minute containing a double leap second. ngl there has never been such a minute 🙈 iconic in theory, never happened in practice.

---

📝 NOTES 📝

🌈 ISO 8601 week dates 🌈

%G, %g, and %V give you values calculated from the week-based year as defined by the ISO 8601 standard. the system, bestie:

• weeks start on a Monday
• they're numbered from 01 (first week) up to 52 or 53 (last week)
• week 1 is the first week where four or more days fall within the new year

and synonymously, week 01 is either: the first week of the year that contains a Thursday, OR the week that has 4 January in it. two ways of saying the same slay.

when three or fewer days of the first calendar week of the new year fall within that year, the ISO 8601 week-based system counts those days as part of week 52 or 53 of the preceding year. it's giving ✨time is a social construct✨.

example time: 1 January 2010 is a Friday, which means just three days of that calendar week fall in 2010. so the ISO 8601 week-based system considers those days to be part of week 53 (%V) of the year 2009 (%G). week 01 of ISO 8601 year 2010 starts on Monday, 4 January 2010. same energy: the first two days of January 2011 are considered part of week 52 of the year 2010. the calendar said new year, ISO 8601 said not yet bestie 😭

💜 glibc notes 💜

glibc provides some extensions for conversion specifications. (these extensions are not specified in POSIX.1-2001, but a few other systems provide similar features.) so basically glibc does a lil extra 🫶

between the % character and the conversion specifier character, you can specify an optional flag and field width. (and these come before the E or O modifiers, if present.)

the flag characters that are permitted:

• _ — (underscore) Pad a numeric result string with spaces.
• - — (dash) Do not pad a numeric result string.
• 0 — Pad a numeric result string with zeros even if the conversion specifier character uses space-padding by default.
• ^ — Convert alphabetic characters in result string to uppercase.
• # — Swap the case of the result string. (This flag works only with certain conversion specifier characters, and of these, it is only really useful with %Z.)

after the (possibly absent) flag, an optional decimal width specifier may follow. if the natural size of the field is smaller than this width, then the result string is padded (on the left) to the specified width. left-padding padded-left, we love a consistent queen 💅

---

💀 BUGS 💀

if the output string would exceed max bytes, errno is not set. 💀

this makes it impossible to distinguish this error case from cases where the format string legitimately produces a zero-length output string. yeah. they're the same picture. and POSIX.1-2001 doesn't specify any errno settings for strftime(), so like, that's just how it is bestie.

also: some buggy versions of gcc(1) complain about the use of %c:

warning: `%c' yields only last 2 digits of year in some locales.

of course programmers are encouraged to use %c, as it gives the preferred date and time representation — like, that's the whole point, it's the main character 🫶. one meets all kinds of strange obfuscations to circumvent this gcc(1) problem. a relatively clean one is to add an intermediate function:

size_t
my_strftime(char *s, size_t max, const char *fmt,
            const struct tm *tm)
{
    return strftime(s, max, fmt, tm);
}

nowadays, gcc(1) provides the -Wno-format-y2k option to prevent the warning, so that the above workaround is no longer required. glow up achieved ✨

---

🍓 EXAMPLES 🍓

RFC 2822-compliant date format (with an English locale for %a and %b)

"%a, %d %b %Y %T %z"

RFC 822-compliant date format (with an English locale for %a and %b)

"%a, %d %b %y %T %z"

🧚 Example program 🧚

the program below can be used to experiment with strftime(). little playground, go wild.

some examples of the result string produced by the glibc implementation of strftime() are as follows:

$ ./a.out '%m'
Result string is "11"
$ ./a.out '%5m'
Result string is "00011"
$ ./a.out '%_5m'
Result string is "   11"

⭐ Program source ⭐

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

int
main(int argc, char *argv[])
{
    char outstr[200];
    time_t t;
    struct tm *tmp;

    if (argc != 2) {
        fprintf(stderr, "Usage: %s: <format-string>\n", argv[0]);
        exit(EXIT_FAILURE);
    }

    t = time(NULL);
    tmp = localtime(&t);
    if (tmp == NULL) {
        perror("localtime");
        exit(EXIT_FAILURE);
    }

    if (strftime(outstr, sizeof(outstr), argv[1], tmp) == 0) {
        fprintf(stderr, "strftime returned 0");
        exit(EXIT_FAILURE);
    }

    printf("Result string is \"%s\"\n", outstr);
    exit(EXIT_SUCCESS);
}

---

👑 SEE ALSO 👑

date(1), time(2), ctime(3), nl_langinfo(3), setlocale(3), sprintf(3), strptime(3)

---

and that's the whole page bestie, we did it 🎉 no cap. this is from Linux man-pages 6.20, dated 2026-07-26, and honestly? slayed. go format some timestamps, you're the moment 💖✨🦋