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. The broken-down time structure tm is defined in
18 <time.h>. See also ctime(3).
19 The format specification is a null-terminated string and may contain
21 special character sequences called conversion specifications, each of
22 which is introduced by a '%' character and terminated by some other
23 character known as a conversion specifier character. All other charac‐
24 ter sequences are ordinary character sequences.
25 The characters of ordinary character sequences (including the null
27 byte) are copied verbatim from format to s. However, the characters of
28 conversion specifications are replaced as shown in the list below. In
29 this list, the field(s) employed from the tm structure are also shown.
30 %a The abbreviated name of the day of the week according to the
32 current locale. (Calculated from tm_wday.)
33 %A The full name of the day of the week according to the current
35 locale. (Calculated from tm_wday.)
36 %b The abbreviated month name according to the current locale.
38 (Calculated from tm_mon.)
39 %B The full month name according to the current locale. (Calcu‐
41 lated from tm_mon.)
42 %c The preferred date and time representation for the current
44 locale.
45 %C The century number (year/100) as a 2-digit integer. (SU) (Calcu‐
47 lated from tm_year.)
48 %d The day of the month as a decimal number (range 01 to 31).
50 (Calculated from tm_mday.)
51 %D Equivalent to %m/%d/%y. (Yecch—for Americans only. Americans
53 should note that in other countries %d/%m/%y is rather common.
54 This means that in international context this format is ambigu‐
55 ous and should not be used.) (SU)
56 %e Like %d, the day of the month as a decimal number, but a leading
58 zero is replaced by a space. (SU) (Calculated from tm_mday.)
59 %E Modifier: use alternative format, see below. (SU)
61 %F Equivalent to %Y-%m-%d (the ISO 8601 date format). (C99)
63 %G The ISO 8601 week-based year (see NOTES) with century as a deci‐
65 mal number. The 4-digit year corresponding to the ISO week num‐
66 ber (see %V). This has the same format and value as %Y, except
67 that if the ISO week number belongs to the previous or next
68 year, that year is used instead. (TZ) (Calculated from tm_year,
69 tm_yday, and tm_wday.)
70 %g Like %G, but without century, that is, with a 2-digit year
72 (00–99). (TZ) (Calculated from tm_year, tm_yday, and tm_wday.)
73 %h Equivalent to %b. (SU)
75 %H The hour as a decimal number using a 24-hour clock (range 00 to
77 23). (Calculated from tm_hour.)
78 %I The hour as a decimal number using a 12-hour clock (range 01 to
80 12). (Calculated from tm_hour.)
81 %j The day of the year as a decimal number (range 001 to 366).
83 (Calculated from tm_yday.)
84 %k The hour (24-hour clock) as a decimal number (range 0 to 23);
86 single digits are preceded by a blank. (See also %H.) (Calcu‐
87 lated from tm_hour.) (TZ)
88 %l The hour (12-hour clock) as a decimal number (range 1 to 12);
90 single digits are preceded by a blank. (See also %I.) (Calcu‐
91 lated from tm_hour.) (TZ)
92 %m The month as a decimal number (range 01 to 12). (Calculated
94 from tm_mon.)
95 %M The minute as a decimal number (range 00 to 59). (Calculated
97 from tm_min.)
98 %n A newline character. (SU)
100 %O Modifier: use alternative format, see below. (SU)
102 %p Either "AM" or "PM" according to the given time value, or the
104 corresponding strings for the current locale. Noon is treated
105 as "PM" and midnight as "AM". (Calculated from tm_hour.)
106 %P Like %p but in lowercase: "am" or "pm" or a corresponding string
108 for the current locale. (Calculated from tm_hour.) (GNU)
109 %r The time in a.m. or p.m. notation. In the POSIX locale this is
111 equivalent to %I:%M:%S %p. (SU)
112 %R The time in 24-hour notation (%H:%M). (SU) For a version
114 including the seconds, see %T below.
115 %s The number of seconds since the Epoch, 1970-01-01 00:00:00 +0000
117 (UTC). (TZ) (Calculated from mktime(tm).)
118 %S The second as a decimal number (range 00 to 60). (The range is
120 up to 60 to allow for occasional leap seconds.) (Calculated
121 from tm_sec.)
122 %t A tab character. (SU)
124 %T The time in 24-hour notation (%H:%M:%S). (SU)
126 %u The day of the week as a decimal, range 1 to 7, Monday being 1.
128 See also %w. (Calculated from tm_wday.) (SU)
129 %U The week number of the current year as a decimal number, range
131 00 to 53, starting with the first Sunday as the first day of
132 week 01. See also %V and %W. (Calculated from tm_yday and
133 tm_wday.)
134 %V The ISO 8601 week number (see NOTES) of the current year as a
136 decimal number, range 01 to 53, where week 1 is the first week
137 that has at least 4 days in the new year. See also %U and %W.
138 (Calculated from tm_year, tm_yday, and tm_wday.) (SU)
139 %w The day of the week as a decimal, range 0 to 6, Sunday being 0.
141 See also %u. (Calculated from tm_wday.)
142 %W The week number of the current year as a decimal number, range
144 00 to 53, starting with the first Monday as the first day of
145 week 01. (Calculated from tm_yday and tm_wday.)
146 %x The preferred date representation for the current locale without
148 the time.
149 %X The preferred time representation for the current locale without
151 the date.
152 %y The year as a decimal number without a century (range 00 to 99).
154 (Calculated from tm_year)
155 %Y The year as a decimal number including the century. (Calculated
157 from tm_year)
158 %z The +hhmm or -hhmm numeric timezone (that is, the hour and
160 minute offset from UTC). (SU)
161 %Z The timezone name or abbreviation.
163 %+ The date and time in date(1) format. (TZ) (Not supported in
165 glibc2.)
166 %% A literal '%' character.
168 Some conversion specifications can be modified by preceding the conver‐
170 sion specifier character by the E or O modifier to indicate that an
171 alternative format should be used. If the alternative format or speci‐
172 fication does not exist for the current locale, the behavior will be as
173 if the unmodified conversion specification were used. (SU) The Single
174 UNIX Specification mentions %Ec, %EC, %Ex, %EX, %Ey, %EY, %Od, %Oe,
175 %OH, %OI, %Om, %OM, %OS, %Ou, %OU, %OV, %Ow, %OW, %Oy, where the effect
176 of the O modifier is to use alternative numeric symbols (say, roman
177 numerals), and that of the E modifier is to use a locale-dependent
178 alternative representation.
179
181 Provided that the result string, including the terminating null byte,
182 does not exceed max bytes, strftime() returns the number of bytes
183 (excluding the terminating null byte) placed in the array s. If the
184 length of the result string (including the terminating null byte) would
185 exceed max bytes, then strftime() returns 0, and the contents of the
186 array are undefined.
187 Note that the return value 0 does not necessarily indicate an error.
189 For example, in many locales %p yields an empty string. An empty for‐
190 mat string will likewise yield an empty string.
191
193 The environment variables TZ and LC_TIME are used.
194
196 For an explanation of the terms used in this section, see
197 attributes(7).
198 ┌───────────┬───────────────┬────────────────────┐
200 │Interface │ Attribute │ Value │
201 ├───────────┼───────────────┼────────────────────┤
202 │strftime() │ Thread safety │ MT-Safe env locale │
203 └───────────┴───────────────┴────────────────────┘
205 SVr4, C89, C99. There are strict inclusions between the set of conver‐
206 sions given in ANSI C (unmarked), those given in the Single UNIX Speci‐
207 fication (marked SU), those given in Olson's timezone package (marked
208 TZ), and those given in glibc (marked GNU), except that %+ is not sup‐
209 ported in glibc2. On the other hand glibc2 has several more exten‐
210 sions. POSIX.1 only refers to ANSI C; POSIX.2 describes under date(1)
211 several extensions that could apply to strftime() as well. The %F con‐
212 version is in C99 and POSIX.1-2001.
213 In SUSv2, the %S specifier allowed a range of 00 to 61, to allow for
215 the theoretical possibility of a minute that included a double leap
216 second (there never has been such a minute).
217
219 ISO 8601 week dates
220 %G, %g, and %V yield values calculated from the week-based year defined
221 by the ISO 8601 standard. In this system, weeks start on a Monday, and
222 are numbered from 01, for the first week, up to 52 or 53, for the last
223 week. Week 1 is the first week where four or more days fall within the
224 new year (or, synonymously, week 01 is: the first week of the year that
225 contains a Thursday; or, the week that has 4 January in it). When
226 three of fewer days of the first calendar week of the new year fall
227 within that year, then the ISO 8601 week-based system counts those days
228 as part of week 53 of the preceding year. For example, 1 January 2010
229 is a Friday, meaning that just three days of that calendar week fall in
230 2010. Thus, the ISO 8601 week-based system considers these days to be
231 part of week 53 (%V) of the year 2009 (%G); week 01 of ISO 8601 year
232 2010 starts on Monday, 4 January 2010.
233 Glibc notes
235 Glibc provides some extensions for conversion specifications. (These
236 extensions are not specified in POSIX.1-2001, but a few other systems
237 provide similar features.) Between the '%' character and the conver‐
238 sion specifier character, an optional flag and field width may be spec‐
239 ified. (These precede the E or O modifiers, if present.)
240 The following flag characters are permitted:
242 _ (underscore) Pad a numeric result string with spaces.
244 - (dash) Do not pad a numeric result string.
246 0 Pad a numeric result string with zeros even if the conversion
248 specifier character uses space-padding by default.
249 ^ Convert alphabetic characters in result string to uppercase.
251 # Swap the case of the result string. (This flag works only with
253 certain conversion specifier characters, and of these, it is
254 only really useful with %Z.)
255 An optional decimal width specifier may follow the (possibly absent)
257 flag. If the natural size of the field is smaller than this width,
258 then the result string is padded (on the left) to the specified width.
259
261 If the output string would exceed max bytes, errno is not set. This
262 makes it impossible to distinguish this error case from cases where the
263 format string legitimately produces a zero-length output string.
264 POSIX.1-2001 does not specify any errno settings for strftime().
265 Some buggy versions of gcc(1) complain about the use of %c: warning:
267 `%c' yields only last 2 digits of year in some locales. Of course pro‐
268 grammers are encouraged to use %c, it gives the preferred date and time
269 representation. One meets all kinds of strange obfuscations to circum‐
270 vent this gcc(1) problem. A relatively clean one is to add an interme‐
271 diate function
272 size_t
274 my_strftime(char *s, size_t max, const char *fmt,
275 const struct tm *tm)
276 {
277 return strftime(s, max, fmt, tm);
278 }
279 Nowadays, gcc(1) provides the -Wno-format-y2k option to prevent the
281 warning, so that the above workaround is no longer required.
282
284 RFC 2822-compliant date format (with an English locale for %a and %b)
285 "%a, %d %b %Y %T %z"
287 RFC 822-compliant date format (with an English locale for %a and %b)
289 "%a, %d %b %y %T %z"
291 Example program
293 The program below can be used to experiment with strftime().
294 Some examples of the result string produced by the glibc implementation
296 of strftime() are as follows:
297 $ ./a.out '%m'
299 Result string is "11"
300 $ ./a.out '%5m'
301 Result string is "00011"
302 $ ./a.out '%_5m'
303 Result string is " 11"
304 Program source
306 #include <time.h>
308 #include <stdio.h>
309 #include <stdlib.h>
310 int
312 main(int argc, char *argv[])
313 {
314 char outstr[200];
315 time_t t;
316 struct tm *tmp;
317 t = time(NULL);
319 tmp = localtime(&t);
320 if (tmp == NULL) {
321 perror("localtime");
322 exit(EXIT_FAILURE);
323 }
324 if (strftime(outstr, sizeof(outstr), argv[1], tmp) == 0) {
326 fprintf(stderr, "strftime returned 0");
327 exit(EXIT_FAILURE);
328 }
329 printf("Result string is \"%s\"\n", outstr);
331 exit(EXIT_SUCCESS);
332 }
333
335 date(1), time(2), ctime(3), setlocale(3), sprintf(3), strptime(3)
336
338 This page is part of release 4.15 of the Linux man-pages project. A
339 description of the project, information about reporting bugs, and the
340 latest version of this page, can be found at
341 https://www.kernel.org/doc/man-pages/.
342GNU 2017-09-15 STRFTIME(3)