1strftime(3) Library Functions Manual strftime(3)
2
6 strftime - format date and time
7
9 Standard C library (libc, -lc)
10
12 #include <time.h>
13 size_t strftime(char s[restrict .max], size_t max,
15 const char *restrict format,
16 const struct tm *restrict tm);
17 size_t strftime_l(char s[restrict .max], size_t max,
19 const char *restrict format,
20 const struct tm *restrict tm,
21 locale_t locale);
22
24 The strftime() function formats the broken-down time tm according to
25 the format specification format and places the result in the character
26 array s of size max. The broken-down time structure tm is defined in
27 <time.h>. See also ctime(3).
28 The format specification is a null-terminated string and may contain
30 special character sequences called conversion specifications, each of
31 which is introduced by a '%' character and terminated by some other
32 character known as a conversion specifier character. All other charac‐
33 ter sequences are ordinary character sequences.
34 The characters of ordinary character sequences (including the null
36 byte) are copied verbatim from format to s. However, the characters of
37 conversion specifications are replaced as shown in the list below. In
38 this list, the field(s) employed from the tm structure are also shown.
39 %a The abbreviated name of the day of the week according to the
41 current locale. (Calculated from tm_wday.) (The specific names
42 used in the current locale can be obtained by calling nl_lang‐
43 info(3) with ABDAY_{1–7} as an argument.)
44 %A The full name of the day of the week according to the current
46 locale. (Calculated from tm_wday.) (The specific names used in
47 the current locale can be obtained by calling nl_langinfo(3)
48 with DAY_{1–7} as an argument.)
49 %b The abbreviated month name according to the current locale.
51 (Calculated from tm_mon.) (The specific names used in the cur‐
52 rent locale can be obtained by calling nl_langinfo(3) with AB‐
53 MON_{1–12} as an argument.)
54 %B The full month name according to the current locale. (Calcu‐
56 lated from tm_mon.) (The specific names used in the current lo‐
57 cale can be obtained by calling nl_langinfo(3) with MON_{1–12}
58 as an argument.)
59 %c The preferred date and time representation for the current lo‐
61 cale. (The specific format used in the current locale can be
62 obtained by calling nl_langinfo(3) with D_T_FMT as an argument
63 for the %c conversion specification, and with ERA_D_T_FMT for
64 the %Ec conversion specification.) (In the POSIX locale this is
65 equivalent to %a %b %e %H:%M:%S %Y.)
66 %C The century number (year/100) as a 2-digit integer. (SU) (The
68 %EC conversion specification corresponds to the name of the
69 era.) (Calculated from tm_year.)
70 %d The day of the month as a decimal number (range 01 to 31).
72 (Calculated from tm_mday.)
73 %D Equivalent to %m/%d/%y. (Yecch—for Americans only. Americans
75 should note that in other countries %d/%m/%y is rather common.
76 This means that in international context this format is ambigu‐
77 ous and should not be used.) (SU)
78 %e Like %d, the day of the month as a decimal number, but a leading
80 zero is replaced by a space. (SU) (Calculated from tm_mday.)
81 %E Modifier: use alternative ("era-based") format, see below. (SU)
83 %F Equivalent to %Y-%m-%d (the ISO 8601 date format). (C99)
85 %G The ISO 8601 week-based year (see NOTES) with century as a deci‐
87 mal number. The 4-digit year corresponding to the ISO week num‐
88 ber (see %V). This has the same format and value as %Y, except
89 that if the ISO week number belongs to the previous or next
90 year, that year is used instead. (TZ) (Calculated from tm_year,
91 tm_yday, and tm_wday.)
92 %g Like %G, but without century, that is, with a 2-digit year
94 (00–99). (TZ) (Calculated from tm_year, tm_yday, and tm_wday.)
95 %h Equivalent to %b. (SU)
97 %H The hour as a decimal number using a 24-hour clock (range 00 to
99 23). (Calculated from tm_hour.)
100 %I The hour as a decimal number using a 12-hour clock (range 01 to
102 12). (Calculated from tm_hour.)
103 %j The day of the year as a decimal number (range 001 to 366).
105 (Calculated from tm_yday.)
106 %k The hour (24-hour clock) as a decimal number (range 0 to 23);
108 single digits are preceded by a blank. (See also %H.) (Calcu‐
109 lated from tm_hour.) (TZ)
110 %l The hour (12-hour clock) as a decimal number (range 1 to 12);
112 single digits are preceded by a blank. (See also %I.) (Calcu‐
113 lated from tm_hour.) (TZ)
114 %m The month as a decimal number (range 01 to 12). (Calculated
116 from tm_mon.)
117 %M The minute as a decimal number (range 00 to 59). (Calculated
119 from tm_min.)
120 %n A newline character. (SU)
122 %O Modifier: use alternative numeric symbols, see below. (SU)
124 %p Either "AM" or "PM" according to the given time value, or the
126 corresponding strings for the current locale. Noon is treated
127 as "PM" and midnight as "AM". (Calculated from tm_hour.) (The
128 specific string representations used for "AM" and "PM" in the
129 current locale can be obtained by calling nl_langinfo(3) with
130 AM_STR and PM_STR, respectively.)
131 %P Like %p but in lowercase: "am" or "pm" or a corresponding string
133 for the current locale. (Calculated from tm_hour.) (GNU)
134 %r The time in a.m. or p.m. notation. (SU) (The specific format
136 used in the current locale can be obtained by calling nl_lang‐
137 info(3) with T_FMT_AMPM as an argument.) (In the POSIX locale
138 this is equivalent to %I:%M:%S %p.)
139 %R The time in 24-hour notation (%H:%M). (SU) For a version in‐
141 cluding the seconds, see %T below.
142 %s The number of seconds since the Epoch, 1970-01-01 00:00:00 +0000
144 (UTC). (TZ) (Calculated from mktime(tm).)
145 %S The second as a decimal number (range 00 to 60). (The range is
147 up to 60 to allow for occasional leap seconds.) (Calculated
148 from tm_sec.)
149 %t A tab character. (SU)
151 %T The time in 24-hour notation (%H:%M:%S). (SU)
153 %u The day of the week as a decimal, range 1 to 7, Monday being 1.
155 See also %w. (Calculated from tm_wday.) (SU)
156 %U The week number of the current year as a decimal number, range
158 00 to 53, starting with the first Sunday as the first day of
159 week 01. See also %V and %W. (Calculated from tm_yday and
160 tm_wday.)
161 %V The ISO 8601 week number (see NOTES) of the current year as a
163 decimal number, range 01 to 53, where week 1 is the first week
164 that has at least 4 days in the new year. See also %U and %W.
165 (Calculated from tm_year, tm_yday, and tm_wday.) (SU)
166 %w The day of the week as a decimal, range 0 to 6, Sunday being 0.
168 See also %u. (Calculated from tm_wday.)
169 %W The week number of the current year as a decimal number, range
171 00 to 53, starting with the first Monday as the first day of
172 week 01. (Calculated from tm_yday and tm_wday.)
173 %x The preferred date representation for the current locale without
175 the time. (The specific format used in the current locale can
176 be obtained by calling nl_langinfo(3) with D_FMT as an argument
177 for the %x conversion specification, and with ERA_D_FMT for the
178 %Ex conversion specification.) (In the POSIX locale this is
179 equivalent to %m/%d/%y.)
180 %X The preferred time representation for the current locale without
182 the date. (The specific format used in the current locale can
183 be obtained by calling nl_langinfo(3) with T_FMT as an argument
184 for the %X conversion specification, and with ERA_T_FMT for the
185 %EX conversion specification.) (In the POSIX locale this is
186 equivalent to %H:%M:%S.)
187 %y The year as a decimal number without a century (range 00 to 99).
189 (The %Ey conversion specification corresponds to the year since
190 the beginning of the era denoted by the %EC conversion specifi‐
191 cation.) (Calculated from tm_year)
192 %Y The year as a decimal number including the century. (The %EY
194 conversion specification corresponds to the full alternative
195 year representation.) (Calculated from tm_year)
196 %z The +hhmm or -hhmm numeric timezone (that is, the hour and
198 minute offset from UTC). (SU)
199 %Z The timezone name or abbreviation.
201 %+ The date and time in date(1) format. (TZ) (Not supported in
203 glibc2.)
204 %% A literal '%' character.
206 Some conversion specifications can be modified by preceding the conver‐
208 sion specifier character by the E or O modifier to indicate that an al‐
209 ternative format should be used. If the alternative format or specifi‐
210 cation does not exist for the current locale, the behavior will be as
211 if the unmodified conversion specification were used. (SU) The Single
212 UNIX Specification mentions %Ec, %EC, %Ex, %EX, %Ey, %EY, %Od, %Oe,
213 %OH, %OI, %Om, %OM, %OS, %Ou, %OU, %OV, %Ow, %OW, %Oy, where the effect
214 of the O modifier is to use alternative numeric symbols (say, roman nu‐
215 merals), and that of the E modifier is to use a locale-dependent alter‐
216 native representation. The rules governing date representation with
217 the E modifier can be obtained by supplying ERA as an argument to a
218 nl_langinfo(3). One example of such alternative forms is the Japanese
219 era calendar scheme in the ja_JP glibc locale.
220 strftime_l() is equivalent to strftime(), except it uses the specified
222 locale instead of the current locale. The behaviour is undefined if
223 locale is invalid or LC_GLOBAL_LOCALE.
224
226 Provided that the result string, including the terminating null byte,
227 does not exceed max bytes, strftime() returns the number of bytes (ex‐
228 cluding the terminating null byte) placed in the array s. If the
229 length of the result string (including the terminating null byte) would
230 exceed max bytes, then strftime() returns 0, and the contents of the
231 array are undefined.
232 Note that the return value 0 does not necessarily indicate an error.
234 For example, in many locales %p yields an empty string. An empty for‐
235 mat string will likewise yield an empty string.
236
238 The environment variables TZ and LC_TIME are used.
239
241 For an explanation of the terms used in this section, see at‐
242 tributes(7).
243 ┌─────────────────────────────────┬───────────────┬────────────────────┐
245 │Interface │ Attribute │ Value │
246 ├─────────────────────────────────┼───────────────┼────────────────────┤
247 │strftime(), strftime_l() │ Thread safety │ MT-Safe env locale │
248 └─────────────────────────────────┴───────────────┴────────────────────┘
249
251 strftime()
252 C11, POSIX.1-2008.
253 strftime_l()
255 POSIX.1-2008.
256
258 strftime()
259 SVr4, C89.
260 strftime_l()
262 POSIX.1-2008.
263 There are strict inclusions between the set of conversions given in AN‐
265 SI C (unmarked), those given in the Single UNIX Specification (marked
266 SU), those given in Olson's timezone package (marked TZ), and those
267 given in glibc (marked GNU), except that %+ is not supported in glibc2.
268 On the other hand glibc2 has several more extensions. POSIX.1 only
269 refers to ANSI C; POSIX.2 describes under date(1) several extensions
270 that could apply to strftime() as well. The %F conversion is in C99
271 and POSIX.1-2001.
272 In SUSv2, the %S specifier allowed a range of 00 to 61, to allow for
274 the theoretical possibility of a minute that included a double leap
275 second (there never has been such a minute).
276
278 ISO 8601 week dates
279 %G, %g, and %V yield values calculated from the week-based year defined
280 by the ISO 8601 standard. In this system, weeks start on a Monday, and
281 are numbered from 01, for the first week, up to 52 or 53, for the last
282 week. Week 1 is the first week where four or more days fall within the
283 new year (or, synonymously, week 01 is: the first week of the year that
284 contains a Thursday; or, the week that has 4 January in it). When
285 three or fewer days of the first calendar week of the new year fall
286 within that year, then the ISO 8601 week-based system counts those days
287 as part of week 52 or 53 of the preceding year. For example, 1 January
288 2010 is a Friday, meaning that just three days of that calendar week
289 fall in 2010. Thus, the ISO 8601 week-based system considers these
290 days to be part of week 53 (%V) of the year 2009 (%G); week 01 of
291 ISO 8601 year 2010 starts on Monday, 4 January 2010. Similarly, the
292 first two days of January 2011 are considered to be part of week 52 of
293 the year 2010.
294 glibc notes
296 glibc provides some extensions for conversion specifications. (These
297 extensions are not specified in POSIX.1-2001, but a few other systems
298 provide similar features.) Between the '%' character and the conver‐
299 sion specifier character, an optional flag and field width may be spec‐
300 ified. (These precede the E or O modifiers, if present.)
301 The following flag characters are permitted:
303 _ (underscore) Pad a numeric result string with spaces.
305 - (dash) Do not pad a numeric result string.
307 0 Pad a numeric result string with zeros even if the conversion
309 specifier character uses space-padding by default.
310 ^ Convert alphabetic characters in result string to uppercase.
312 # Swap the case of the result string. (This flag works only with
314 certain conversion specifier characters, and of these, it is on‐
315 ly really useful with %Z.)
316 An optional decimal width specifier may follow the (possibly absent)
318 flag. If the natural size of the field is smaller than this width,
319 then the result string is padded (on the left) to the specified width.
320
322 If the output string would exceed max bytes, errno is not set. This
323 makes it impossible to distinguish this error case from cases where the
324 format string legitimately produces a zero-length output string.
325 POSIX.1-2001 does not specify any errno settings for strftime().
326 Some buggy versions of gcc(1) complain about the use of %c: warning:
328 `%c' yields only last 2 digits of year in some locales. Of course pro‐
329 grammers are encouraged to use %c, as it gives the preferred date and
330 time representation. One meets all kinds of strange obfuscations to
331 circumvent this gcc(1) problem. A relatively clean one is to add an
332 intermediate function
333 size_t
335 my_strftime(char *s, size_t max, const char *fmt,
336 const struct tm *tm)
337 {
338 return strftime(s, max, fmt, tm);
339 }
340 Nowadays, gcc(1) provides the -Wno-format-y2k option to prevent the
342 warning, so that the above workaround is no longer required.
343
345 RFC 2822-compliant date format (with an English locale for %a and %b)
346 "%a, %d %b %Y %T %z"
348 RFC 822-compliant date format (with an English locale for %a and %b)
350 "%a, %d %b %y %T %z"
352 Example program
354 The program below can be used to experiment with strftime().
355 Some examples of the result string produced by the glibc implementation
357 of strftime() are as follows:
358 $ ./a.out '%m'
360 Result string is "11"
361 $ ./a.out '%5m'
362 Result string is "00011"
363 $ ./a.out '%_5m'
364 Result string is " 11"
365 Program source
367 #include <stdio.h>
369 #include <stdlib.h>
370 #include <time.h>
371 int
373 main(int argc, char *argv[])
374 {
375 char outstr[200];
376 time_t t;
377 struct tm *tmp;
378 t = time(NULL);
380 tmp = localtime(&t);
381 if (tmp == NULL) {
382 perror("localtime");
383 exit(EXIT_FAILURE);
384 }
385 if (strftime(outstr, sizeof(outstr), argv[1], tmp) == 0) {
387 fprintf(stderr, "strftime returned 0");
388 exit(EXIT_FAILURE);
389 }
390 printf("Result string is \"%s\"\n", outstr);
392 exit(EXIT_SUCCESS);
393 }
394
396 date(1), time(2), ctime(3), nl_langinfo(3), setlocale(3), sprintf(3),
397 strptime(3)
398Linux man-pages 6.04 2023-03-30 strftime(3)