1STRFTIME(3) Linux Programmer's Manual STRFTIME(3)
2
6 strftime - format date and time
7
9 #include <time.h>
10 size_t strftime(char *s, size_t max, const char *format,
12 const struct tm *tm);
13
15 The strftime() function formats the broken-down time tm according to
16 the format specification format and places the result in the character
17 array s of size max.
18 The format specification is a null-terminated string and may contain
20 special character sequences called conversion specifications, each of
21 which is introduced by a '%' character and terminated by some other
22 character known as a conversion specifier character. All other charac‐
23 ter sequences are ordinary character sequences.
24 The characters of ordinary character sequences (including the null
26 byte) are copied verbatim from format to s. However, the characters of
27 conversion specifications are replaced as follows:
28 %a The abbreviated weekday name according to the current locale.
30 %A The full weekday name according to the current locale.
32 %b The abbreviated month name according to the current locale.
34 %B The full month name according to the current locale.
36 %c The preferred date and time representation for the current
38 locale.
39 %C The century number (year/100) as a 2-digit integer. (SU)
41 %d The day of the month as a decimal number (range 01 to 31).
43 %D Equivalent to %m/%d/%y. (Yecch—for Americans only. Americans
45 should note that in other countries %d/%m/%y is rather common.
46 This means that in international context this format is ambigu‐
47 ous and should not be used.) (SU)
48 %e Like %d, the day of the month as a decimal number, but a leading
50 zero is replaced by a space. (SU)
51 %E Modifier: use alternative format, see below. (SU)
53 %F Equivalent to %Y-%m-%d (the ISO 8601 date format). (C99)
55 %G The ISO 8601 week-based year (see NOTES) with century as a deci‐
57 mal number. The 4-digit year corresponding to the ISO week num‐
58 ber (see %V). This has the same format and value as %Y, except
59 that if the ISO week number belongs to the previous or next
60 year, that year is used instead. (TZ)
61 %g Like %G, but without century, that is, with a 2-digit year
63 (00-99). (TZ)
64 %h Equivalent to %b. (SU)
66 %H The hour as a decimal number using a 24-hour clock (range 00 to
68 23).
69 %I The hour as a decimal number using a 12-hour clock (range 01 to
71 12).
72 %j The day of the year as a decimal number (range 001 to 366).
74 %k The hour (24-hour clock) as a decimal number (range 0 to 23);
76 single digits are preceded by a blank. (See also %H.) (TZ)
77 %l The hour (12-hour clock) as a decimal number (range 1 to 12);
79 single digits are preceded by a blank. (See also %I.) (TZ)
80 %m The month as a decimal number (range 01 to 12).
82 %M The minute as a decimal number (range 00 to 59).
84 %n A newline character. (SU)
86 %O Modifier: use alternative format, see below. (SU)
88 %p Either "AM" or "PM" according to the given time value, or the
90 corresponding strings for the current locale. Noon is treated
91 as "PM" and midnight as "AM".
92 %P Like %p but in lowercase: "am" or "pm" or a corresponding string
94 for the current locale. (GNU)
95 %r The time in a.m. or p.m. notation. In the POSIX locale this is
97 equivalent to %I:%M:%S %p. (SU)
98 %R The time in 24-hour notation (%H:%M). (SU) For a version
100 including the seconds, see %T below.
101 %s The number of seconds since the Epoch, 1970-01-01 00:00:00 +0000
103 (UTC). (TZ)
104 %S The second as a decimal number (range 00 to 60). (The range is
106 up to 60 to allow for occasional leap seconds.)
107 %t A tab character. (SU)
109 %T The time in 24-hour notation (%H:%M:%S). (SU)
111 %u The day of the week as a decimal, range 1 to 7, Monday being 1.
113 See also %w. (SU)
114 %U The week number of the current year as a decimal number, range
116 00 to 53, starting with the first Sunday as the first day of
117 week 01. See also %V and %W.
118 %V The ISO 8601 week number (see NOTES) of the current year as a
120 decimal number, range 01 to 53, where week 1 is the first week
121 that has at least 4 days in the new year. See also %U and %W.
122 (SU)
123 %w The day of the week as a decimal, range 0 to 6, Sunday being 0.
125 See also %u.
126 %W The week number of the current year as a decimal number, range
128 00 to 53, starting with the first Monday as the first day of
129 week 01.
130 %x The preferred date representation for the current locale without
132 the time.
133 %X The preferred time representation for the current locale without
135 the date.
136 %y The year as a decimal number without a century (range 00 to 99).
138 %Y The year as a decimal number including the century.
140 %z The +hhmm or -hhmm numeric timezone (that is, the hour and
142 minute offset from UTC). (SU)
143 %Z The timezone name or abbreviation.
145 %+ The date and time in date(1) format. (TZ) (Not supported in
147 glibc2.)
148 %% A literal '%' character.
150 Some conversion specifications can be modified by preceding the conver‐
152 sion specifier character by the E or O modifier to indicate that an
153 alternative format should be used. If the alternative format or speci‐
154 fication does not exist for the current locale, the behavior will be as
155 if the unmodified conversion specification were used. (SU) The Single
156 UNIX Specification mentions %Ec, %EC, %Ex, %EX, %Ey, %EY, %Od, %Oe,
157 %OH, %OI, %Om, %OM, %OS, %Ou, %OU, %OV, %Ow, %OW, %Oy, where the effect
158 of the O modifier is to use alternative numeric symbols (say, roman
159 numerals), and that of the E modifier is to use a locale-dependent
160 alternative representation.
161 The broken-down time structure tm is defined in <time.h>. See also
163 ctime(3).
164
166 Provided that the result string, including the terminating null byte,
167 does not exceed max bytes, strftime() returns the number of bytes
168 (excluding the terminating null byte) placed in the array s. If the
169 length of the result string (including the terminating null byte) would
170 exceed max bytes, then strftime() returns 0, and the contents of the
171 array are undefined. (This behavior applies since at least libc 4.4.4;
172 very old versions of libc, such as libc 4.4.1, would return max if the
173 array was too small.)
174 Note that the return value 0 does not necessarily indicate an error.
176 For example, in many locales %p yields an empty string. An empty for‐
177 mat string will likewise yield an empty string.
178
180 The environment variables TZ and LC_TIME are used.
181
183 SVr4, C89, C99. There are strict inclusions between the set of conver‐
184 sions given in ANSI C (unmarked), those given in the Single UNIX Speci‐
185 fication (marked SU), those given in Olson's timezone package (marked
186 TZ), and those given in glibc (marked GNU), except that %+ is not sup‐
187 ported in glibc2. On the other hand glibc2 has several more exten‐
188 sions. POSIX.1 only refers to ANSI C; POSIX.2 describes under date(1)
189 several extensions that could apply to strftime() as well. The %F con‐
190 version is in C99 and POSIX.1-2001.
191 In SUSv2, the %S specifier allowed a range of 00 to 61, to allow for
193 the theoretical possibility of a minute that included a double leap
194 second (there never has been such a minute).
195
197 ISO 8601 week dates
198 %G, %g, and %V yield values calculated from the week-based year defined
199 by the ISO 8601 standard. In this system, weeks start on a Monday, and
200 are numbered from 01, for the first week, up to 52 or 53, for the last
201 week. Week 1 is the first week where four or more days fall within the
202 new year (or, synonymously, week 01 is: the first week of the year that
203 contains a Thursday; or, the week that has 4 January in it). When
204 three of fewer days of the first calendar week of the new year fall
205 within that year, then the ISO 8601 week-based system counts those days
206 as part of week 53 of the preceding year. For example, 1 January 2010
207 is a Friday, meaning that just three days of that calendar week fall in
208 2010. Thus, the ISO 8601 week-based system considers these days to be
209 part of week 53 (%V) of the year 2009 (%G); week 01 of ISO 8601 year
210 2010 starts on Monday, 4 January 2010.
211 Glibc notes
213 Glibc provides some extensions for conversion specifications. (These
214 extensions are not specified in POSIX.1-2001, but a few other systems
215 provide similar features.) Between the '%' character and the conver‐
216 sion specifier character, an optional flag and field width may be spec‐
217 ified. (These precede the E or O modifiers, if present.)
218 The following flag characters are permitted:
220 _ (underscore) Pad a numeric result string with spaces.
222 - (dash) Do not pad a numeric result string.
224 0 Pad a numeric result string with zeros even if the conversion
226 specifier character uses space-padding by default.
227 ^ Convert alphabetic characters in result string to upper case.
229 # Swap the case of the result string. (This flag works only with
231 certain conversion specifier characters, and of these, it is
232 only really useful with %Z.)
233 An optional decimal width specifier may follow the (possibly absent)
235 flag. If the natural size of the field is smaller than this width,
236 then the result string is padded (on the left) to the specified width.
237
239 If the output string would exceed max bytes, errno is not set. This
240 makes it impossible to distinguish this error case from cases where the
241 format string legitimately produces a zero-length output string.
242 POSIX.1-2001 does not specify any errno settings for strftime().
243 Some buggy versions of gcc(1) complain about the use of %c: warning:
245 `%c' yields only last 2 digits of year in some locales. Of course pro‐
246 grammers are encouraged to use %c, it gives the preferred date and time
247 representation. One meets all kinds of strange obfuscations to circum‐
248 vent this gcc(1) problem. A relatively clean one is to add an interme‐
249 diate function
250 size_t
252 my_strftime(char *s, size_t max, const char *fmt,
253 const struct tm *tm)
254 {
255 return strftime(s, max, fmt, tm);
256 }
257 Nowadays, gcc(1) provides the -Wno-format-y2k option to prevent the
259 warning, so that the above workaround is no longer required.
260
262 RFC 2822-compliant date format (with an English locale for %a and %b)
263 "%a, %d %b %Y %T %z"
265 RFC 822-compliant date format (with an English locale for %a and %b)
267 "%a, %d %b %y %T %z"
269 Example program
271 The program below can be used to experiment with strftime().
272 Some examples of the result string produced by the glibc implementation
274 of strftime() are as follows:
275 $ ./a.out '%m'
277 Result string is "11"
278 $ ./a.out '%5m'
279 Result string is "00011"
280 $ ./a.out '%_5m'
281 Result string is " 11"
282 Here's the program source:
284 #include <time.h>
286 #include <stdio.h>
287 #include <stdlib.h>
288 int
290 main(int argc, char *argv[])
291 {
292 char outstr[200];
293 time_t t;
294 struct tm *tmp;
295 t = time(NULL);
297 tmp = localtime(&t);
298 if (tmp == NULL) {
299 perror("localtime");
300 exit(EXIT_FAILURE);
301 }
302 if (strftime(outstr, sizeof(outstr), argv[1], tmp) == 0) {
304 fprintf(stderr, "strftime returned 0");
305 exit(EXIT_FAILURE);
306 }
307 printf("Result string is \"%s\"\n", outstr);
309 exit(EXIT_SUCCESS);
310 }
311
313 date(1), time(2), ctime(3), setlocale(3), sprintf(3), strptime(3)
314
316 This page is part of release 3.53 of the Linux man-pages project. A
317 description of the project, and information about reporting bugs, can
318 be found at http://www.kernel.org/doc/man-pages/.
319GNU 2013-06-28 STRFTIME(3)