strftime(3)
NAME
strftime - format date and time
SYNOPSIS
#include <time.h>
size_t strftime(char *s, size_t max, const char *format,
const struct tm *tm);
DESCRIPTION
The strftime() function formats the broken-down time tm according to the format specification format and places the result in the character array s of size max. The broken-down time structure tm is defined in <time.h>. See also ctime(3). The format specification is a null-terminated string and may contain special character sequences called conversion specifications, each of which is introduced by a '%' character and terminated by some other character known as a conversion specifier character. All other character sequences are ordinary character sequences. The characters of ordinary character sequences (including the null byte) are copied verbatim from format to s. However, the characters of conversion specifications are replaced as shown in the list below. In this list, the field(s) employed from the tm structure are also shown. %a The abbreviated name of the day of the week according to the current locale. (Calculated from tm_wday.) %A The full name of the day of the week according to the current locale. (Calculated from tm_wday.) %b The abbreviated month name according to the current locale. (Calculated from tm_mon.) %B The full month name according to the current locale. (Calculated from tm_mon.) %c The preferred date and time representation for the current locale. %C The century number (year/100) as a 2-digit integer. (SU) (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 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 format, 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.) %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. In the POSIX locale this is equivalent to %I:%M:%S %p. (SU) %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. %X The preferred time representation for the current locale without the date. %y The year as a decimal number without a century (range 00 to 99). (Calculated from tm_year) %Y The year as a decimal number including the century. (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. Some conversion specifications can be modified by preceding the conversion specifier character by the E or O modifier to indicate that an alternative format should be used. If the alternative format or specification does not 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 effect of the O modifier is to use alternative numeric symbols (say, roman numerals), and that of the E modifier is to use a locale- dependent alternative representation.
RETURN VALUE
Provided that the result string, including the terminating null byte, does not exceed max bytes, strftime() returns the number of bytes (excluding the terminating null byte) placed in the array s. If the length of the result string (including the terminating null byte) would exceed max bytes, then strftime() returns 0, and the contents of the array are undefined. Note that the return value 0 does not necessarily indicate an error. For example, in many locales %p yields an empty string. An empty format string will likewise yield an empty string.
ENVIRONMENT
The environment variables TZ and LC_TIME are used.
ATTRIBUTES
For an explanation of the terms used in this section, see attributes(7). Interface Attribute Value strftime() Thread safety MT-Safe env locale
CONFORMING TO
SVr4, C89, C99. There are strict inclusions between the set of conversions given in ANSI C (unmarked), those given in the Single UNIX Specification (marked SU), those given in Olson's timezone package (marked TZ), and those given in glibc (marked GNU), except that %+ is not supported in glibc2. On the other hand glibc2 has several more extensions. POSIX.1 only refers to ANSI C; POSIX.2 describes under date(1) several extensions that could apply to strftime() as well. The %F conversion is in C99 and POSIX.1-2001. In SUSv2, the %S specifier allowed a range of 00 to 61, to allow for the theoretical possibility of a minute that included a double leap second (there never has been such a minute).
NOTES
ISO 8601 week dates
%G, %g, and %V yield values calculated from the week-based year defined
by the ISO 8601 standard. In this system, weeks start on a Monday, and
are numbered from 01, for the first week, up to 52 or 53, for the last
week. Week 1 is the first week where four or more days fall within the
new year (or, synonymously, week 01 is: the first week of the year that
contains a Thursday; or, the week that has 4 January in it). When
three of fewer days of the first calendar week of the new year fall
within that year, then the ISO 8601 week-based system counts those days
as part of week 53 of the preceding year. For example, 1 January 2010
is a Friday, meaning that just three days of that calendar week fall in
2010. Thus, the ISO 8601 week-based system considers these 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.
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.) Between the '%' character and the
conversion specifier character, an optional flag and field width may be
specified. (These precede the E or O modifiers, if present.)
The following flag characters 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.)
An optional decimal width specifier may follow the (possibly absent)
flag. 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.
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. POSIX.1-2001 does not specify any errno settings for strftime(). 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, it gives the preferred date and time representation. 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.
EXAMPLE
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().
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 <time.h>
#include <stdio.h>
#include <stdlib.h>
int
main(int argc, char *argv[])
{
char outstr[200];
time_t t;
struct tm *tmp;
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), setlocale(3), sprintf(3), strptime(3)
COLOPHON
This page is part of release 4.09 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at https://www.kernel.org/doc/man-pages/.
Opportunity
Personal Opportunity - Free software gives you access to billions of dollars of software at no cost. Use this software for your business, personal use or to develop a profitable skill. Access to source code provides access to a level of capabilities/information that companies protect though copyrights. Open source is a core component of the Internet and it is available to you. Leverage the billions of dollars in resources and capabilities to build a career, establish a business or change the world. The potential is endless for those who understand the opportunity.
Business Opportunity - Goldman Sachs, IBM and countless large corporations are leveraging open source to reduce costs, develop products and increase their bottom lines. Learn what these companies know about open source and how open source can give you the advantage.
Free Software
Free Software provides computer programs and capabilities at no cost but more importantly, it provides the freedom to run, edit, contribute to, and share the software. The importance of free software is a matter of access, not price. Software at no cost is a benefit but ownership rights to the software and source code is far more significant.
Free Office Software - The Libre Office suite provides top desktop productivity tools for free. This includes, a word processor, spreadsheet, presentation engine, drawing and flowcharting, database and math applications. Libre Office is available for Linux or Windows.
Free Books
The Free Books Library is a collection of thousands of the most popular public domain books in an online readable format. The collection includes great classical literature and more recent works where the U.S. copyright has expired. These books are yours to read and use without restrictions.
Source Code - Want to change a program or know how it works? Open Source provides the source code for its programs so that anyone can use, modify or learn how to write those programs themselves. Visit the GNU source code repositories to download the source.
Education
Study at Harvard, Stanford or MIT - Open edX provides free online courses from Harvard, MIT, Columbia, UC Berkeley and other top Universities. Hundreds of courses for almost all major subjects and course levels. Open edx also offers some paid courses and selected certifications.
Linux Manual Pages - A man or manual page is a form of software documentation found on Linux/Unix operating systems. Topics covered include computer programs (including library and system calls), formal standards and conventions, and even abstract concepts.
