strftime, strftime_l — convert date and time to a string
#include <time.h>
size_t strftime(char *restrict s, size_t maxsize,
const char *restrict format, const struct tm *restrict timeptr);[CX]size_t strftime_l(char *restrict s, size_t maxsize,
const char *restrict format, const struct tm *restrict timeptr,
locale_t locale);
For strftime(): [CX]
The strftime() function shall place bytes into the array pointed to by s as controlled by the string pointed to by format. The application shall ensure that the format is a character string, beginning and ending in its initial shift state, if any. The format string consists of zero or more conversion specifications and ordinary characters.
Each conversion specification is introduced by the '%' character after which the following appear in sequence:
[CX]
- 0
- The zero character ('0'), which specifies that the character used as the padding character is '0',
- +
- The <plus-sign> character ('+'), which specifies that the character used as the padding character is '0', and that if and only if the field being produced consumes more than four bytes to represent a year (for %F, %G, or %Y) or more than two bytes to represent the year divided by 100 (for %C) then a leading <plus-sign> character shall be included if the year being processed is greater than or equal to zero or a leading <hyphen-minus> character ('-') shall be included if the year is less than zero.
The default padding character is unspecified.
An optional minimum field width. If the converted value, including any leading '+' or '-' sign, has fewer bytes than the minimum field width and the padding character is not the NUL character, the output shall be padded on the left (after any leading '+' or '-' sign) with the padding character.
An optional E or O modifier.
A terminating conversion specifier character that indicates the type of conversion to be applied.
[CX]
All ordinary characters (including the terminating NUL character) are copied unchanged into the array. If copying takes place between objects that overlap, the behavior is undefined. No more than maxsize bytes are placed into the array. Each conversion specifier is replaced by appropriate characters as described in the following list. The appropriate characters are determined using the LC_TIME category of the current locale and by the values of zero or more members of the broken-down time structure pointed to by timeptr, as specified in brackets in the description. If any of the specified values are outside the normal range, the characters stored are unspecified.
[CX]
Local timezone information shall be set as though strftime() called tzset().
The following conversion specifiers shall be supported:
- a
- Replaced by the locale's abbreviated weekday name. [tm_wday]
- A
- Replaced by the locale's full weekday name. [tm_wday]
- b
- Replaced by the locale's abbreviated month name. [tm_mon]
- B
- Replaced by the locale's full month name. [tm_mon]
- c
- Replaced by the locale's appropriate date and time representation. (See the Base Definitions volume of POSIX.1-2024, <time.h>.)
- C
- Replaced by the year divided by 100 and truncated to an integer, as a decimal number. [tm_year]
If a minimum field width is not specified:
If the year is between 0 and 9999 inclusive, two characters shall be placed into the array pointed to by s, including a leading '0' if there would otherwise be only a single digit.
[CX]
[CX]
- d
- Replaced by the day of the month as a decimal number [01,31]. [tm_mday]
- D
- Equivalent to %m/%d/%y. [tm_mon, tm_mday, tm_year]
- e
- Replaced by the day of the month as a decimal number [1,31]; a single digit is preceded by a space. [tm_mday]
- F
- Equivalent to %Y-%m-%d if no flag and no minimum field width are specified. (For years between 1000 and 9999 inclusive this provides the ISO 8601:2019 standard complete representation, extended format date representation of a specific day.) [tm_year, tm_mon, tm_mday]
[CX]
If the minimum field width is specified to be 10, and the year is four digits long, then the output string produced shall match the ISO 8601:2019 standard subclause 4.1.2.2 complete representation, extended format date representation of a specific day. If a + flag is specified, a minimum field width of x is specified, and x-7 bytes are sufficient to hold the digits of the year (not including any needed sign character), then the output shall match the ISO 8601:2019 standard subclause 4.1.2.4 complete representation, expanded format date representation of a specific day.
- g
- Replaced by the last 2 digits of the week-based year (see below) as a decimal number [00,99]. [tm_year, tm_wday, tm_yday]
- G
- Replaced by the week-based year (see below) as a decimal number (for example, 1977). [tm_year, tm_wday, tm_yday]
[CX]
- h
- Equivalent to %b. [tm_mon]
- H
- Replaced by the hour (24-hour clock) as a decimal number [00,23]. [tm_hour]
- I
- Replaced by the hour (12-hour clock) as a decimal number [01,12]. [tm_hour]
- j
- Replaced by the day of the year as a decimal number [001,366]. [tm_yday]
- m
- Replaced by the month as a decimal number [01,12]. [tm_mon]
- M
- Replaced by the minute as a decimal number [00,59]. [tm_min]
- n
- Replaced by a <newline>.
- p
- Replaced by the locale's equivalent of either a.m. or p.m. [tm_hour]
- r
- Replaced by the time in 12-hour clock notation; [CX]
[tm_hour, tm_min, tm_sec]
- R
- Replaced by the time in 24-hour notation (%H:%M). [tm_hour, tm_min]
- s
- [CX]
- S
- Replaced by the second as a decimal number [00,60]. [tm_sec]
- t
- Replaced by a <tab>.
- T
- Replaced by the time (%H:%M:%S). [tm_hour, tm_min, tm_sec]
- u
- Replaced by the weekday as a decimal number [1,7], with 1 representing Monday. [tm_wday]
- U
- Replaced by the week number of the year as a decimal number [00,53]. The first Sunday of January is the first day of week 1; days in the new year before this are in week 0. [tm_year, tm_wday, tm_yday]
- V
- Replaced by the week number of the year (Monday as the first day of the week) as a decimal number [01,53]. If the week containing 1 January has four or more days in the new year, then it is considered week 1. Otherwise, it is the last week of the previous year, and the next week is week 1. Both January 4th and the first Thursday of January are always in week 1. [tm_year, tm_wday, tm_yday]
- w
- Replaced by the weekday as a decimal number [0,6], with 0 representing Sunday. [tm_wday]
- W
- Replaced by the week number of the year as a decimal number [00,53]. The first Monday of January is the first day of week 1; days in the new year before this are in week 0. [tm_year, tm_wday, tm_yday]
- x
- Replaced by the locale's appropriate date representation. (See the Base Definitions volume of POSIX.1-2024, <time.h>.)
- X
- Replaced by the locale's appropriate time representation. (See the Base Definitions volume of POSIX.1-2024, <time.h>.)
- y
- Replaced by the last two digits of the year as a decimal number [00,99]. [tm_year]
- Y
- Replaced by the year as a decimal number (for example, 1997). [tm_year]
[CX]
- z
- Replaced by the offset from UTC in the ISO 8601:2019 standard format (+hhmm or -hhmm), or by no characters if no timezone is determinable. For example, "-0430" means 4 hours 30 minutes behind UTC (west of Greenwich). [CX]
[tm_isdst, [CX]- Z
- Replaced by the timezone name or abbreviation, or by no bytes if no timezone information exists. [tm_isdst, [CX]
- %
- Replaced by %.
If a conversion specification does not correspond to any of the above, the behavior is undefined.
[CX]
If a struct tm broken-down time structure is created or modified by gmtime() or gmtime_r(), it is unspecified whether the result of the %Z and %z conversion specifiers shall refer to UTC or the current local timezone, when strftime() is called with such a broken-down time structure.
Modified Conversion Specifiers
Some conversion specifiers can be modified by the E or O modifier characters to indicate that an alternative format or specification should be used rather than the one normally used by the unmodified conversion specifier. If the alternative format or specification does not exist for the current locale (see ERA in XBD 7.3.5 LC_TIME), the behavior shall be as if the unmodified conversion specification were used.
- %Ec
- Replaced by the locale's alternative appropriate date and time representation.
- %EC
- Replaced by the name of the base year (period) in the locale's alternative representation.
- %Ex
- Replaced by the locale's alternative date representation.
- %EX
- Replaced by the locale's alternative time representation.
- %Ey
- Replaced by the offset from %EC (year only) in the locale's alternative representation.
- %EY
- Replaced by the full alternative year representation.
- %Ob
- [CX]
- %OB
- [CX]
- %Od
- Replaced by the day of the month, using the locale's alternative numeric symbols, filled as needed with leading zeros if there is any alternative symbol for zero; otherwise, with leading <space> characters.
- %Oe
- Replaced by the day of the month, using the locale's alternative numeric symbols, filled as needed with leading <space> characters.
- %OH
- Replaced by the hour (24-hour clock) using the locale's alternative numeric symbols.
- %OI
- Replaced by the hour (12-hour clock) using the locale's alternative numeric symbols.
- %Om
- Replaced by the month using the locale's alternative numeric symbols.
- %OM
- Replaced by the minutes using the locale's alternative numeric symbols.
- %OS
- Replaced by the seconds using the locale's alternative numeric symbols.
- %Ou
- Replaced by the weekday as a number in the locale's alternative representation (Monday=1).
- %OU
- Replaced by the week number of the year (Sunday as the first day of the week, rules corresponding to %U) using the locale's alternative numeric symbols.
- %OV
- Replaced by the week number of the year (Monday as the first day of the week, rules corresponding to %V) using the locale's alternative numeric symbols.
- %Ow
- Replaced by the number of the weekday (Sunday=0) using the locale's alternative numeric symbols.
- %OW
- Replaced by the week number of the year (Monday as the first day of the week) using the locale's alternative numeric symbols.
- %Oy
- Replaced by the year (offset from %C) using the locale's alternative numeric symbols.
%g, %G, and %V give values according to the ISO 8601:2019 standard week-based year. In this system, weeks begin on a Monday and week 1 of the year is the week that includes January 4th, which is also the week that includes the first Thursday of the year, and is also the first week that contains at least four days in the year. If the first Monday of January is the 2nd, 3rd, or 4th, the preceding days are part of the last week of the preceding year; thus, for Saturday 2nd January 1999, %G is replaced by 1998 and %V is replaced by 53. If December 29th, 30th, or 31st is a Monday, it and any following days are part of week 1 of the following year. Thus, for Tuesday 30th December 1997, %G is replaced by 1998 and %V is replaced by 01.
If a conversion specifier is not one of the above, the behavior is undefined.
[CX]
If successful, these functions shall return the number of bytes placed into the array pointed to by s, not including the terminating NUL character. [CX]
[CX]
- [ERANGE]
- The total number of resulting bytes including the terminating NUL character is more than maxsize.
These functions may fail if:
- [EINVAL]
- The format string includes a %s conversion and the number of seconds since the Epoch would be negative.
- [EOVERFLOW]
- The format string includes a %s conversion and the number of seconds since the Epoch cannot be represented in a time_t.
Getting a Localized Date String
The following example first sets the locale to the user's default. The locale information will be used in the nl_langinfo() and strftime() functions. The nl_langinfo() function returns the localized date string which specifies how the date is laid out. The strftime() function takes this information and, using the tm structure for values, places the date and time information into datestring.
#include <time.h> #include <locale.h> #include <langinfo.h> ... struct tm *tm; char datestring[256]; ... setlocale (LC_ALL, ""); ... strftime (datestring, sizeof(datestring), nl_langinfo (D_T_FMT), tm); ...
A return value of 0 may indicate either success or failure if a format is the empty string or consists of conversion specifications such as %p or %Z that are replaced by no characters in the current locale or because of the current setting of tzname[], respectively. To distinguish between success and failure when strftime() returns 0, an application can set errno to 0 before calling strftime() and test whether errno is 0 afterwards.
The range of values for %S is [00,60] rather than [00,59] to allow for the occasional leap second.
Some of the conversion specifications are duplicates of others. They are included for compatibility with nl_cxtime() and nl_ascxtime(), which were published in Issue 2.
The %C, %F, %G, and %Y format specifiers in strftime() always print full values, but the strptime() %C, %F, and %Y format specifiers only scan two digits (assumed to be the first two digits of a four-digit year) for %C and four digits (assumed to be the entire (four-digit) year) for %F and %Y. This mimics the behavior of printf() and scanf(); that is:
printf("%2d", x = 1000);prints "1000", but:
scanf(%2d", &x);when given "1000" as input will only store 10 in x). Applications using extended ranges of years must be sure that the number of digits specified for scanning years with strptime() matches the number of digits that will actually be present in the input stream. Historic implementations of the %Y conversion specification (with no flags and no minimum field width) produced different output formats. Some always produced at least four digits (with 0 fill for years from 0 through 999) while others only produced the number of digits present in the year (with no fill and no padding). These two forms can be produced with the '0' flag and a minimum field width options using the conversions specifications %04Y and %01Y, respectively. Similarly, because %Y is part of %F, field widths of 10 and 7 (%010F, %07F), respectively, produce the same effect in the year portion of the %F conversion result.
In the past, the C and POSIX standards specified that %F produced an ISO 8601:2019 standard date format, but didn't specify which one. For years in the range [1000,9999], POSIX.1-2024 requires that the output produced match the ISO 8601:2019 standard complete representation extended format (YYYY-MM-DD) and for years greater than 9999 produce output that matches the ISO 8601:2019 standard expanded representation extended format ±Y'-[Ywidth]u".15m''[Ywidth]u"-.15m'YYYY-MM-DD). For years less than 1000, %F is not required to produce an ISO 8601:2019 standard format when used without specifying at least a minimum field width. As stated above, some implementations pad %Y conversions with zeros to four digits, in which case %F produces an ISO 8601:2019 standard format; other implementations do not pad %Y with zeros, in which case %F does not produce an ISO 8601:2019 standard format. To fully meet ISO 8601:2019 standard requirements, the producer and consumer must agree on a date format that has a specific number of bytes reserved to hold the characters used to represent the years that is sufficiently large to hold all values that will be shared. For example, the %+13F conversion specification will produce output matching the format "±YYYYYY-MM-DD" (a leading '+' or '-' sign; a six-digit, 0-filled year; a '-'; a two-digit, leading 0-filled month; another '-'; and the two-digit, leading 0-filled day within the month).
Note that if the year being printed is greater than 9999, the resulting string from the unadorned %F conversion specifications will not conform to the ISO 8601:2019 standard extended format, complete representation for a date and will instead be an extended format, expanded representation (presumably without the required agreement between the date's producer and consumer).
In the C or POSIX locale, the E and O modifiers are ignored and the replacement strings for the following specifiers are:
- %a
- The first three characters of %A.
- %A
- One of Sunday, Monday, ..., Saturday.
- %b
- The first three characters of %B.
- %B
- One of January, February, ..., December.
- %c
- Equivalent to %a %b %e %T %Y.
- %p
- One of AM or PM.
- %r
- Equivalent to %I:%M:%S %p.
- %x
- Equivalent to %m/%d/%y.
- %X
- Equivalent to %T.
- %Z
- Implementation-defined.
The %Y conversion specification to strftime() was frequently assumed to be a four-digit year, but the ISO C standard does not specify that %Y is restricted to any subset of allowed values from the tm_year field. Similarly, the %C conversion specification was assumed to be a two-digit field and the first part of the output from the %F conversion specification was assumed to be a four-digit field. Since tm_year is a signed int with a width of at least 32 bits and time_t is required to have a width of at least 64 bits (in conforming programming environments), these assumptions no longer hold.
POSIX.1-2024 now allows the format specifications %0xC, %0xF, %0xG, and %0xY (where 'x' is a string of decimal digits used to specify printing and scanning of a string of x decimal digits) with leading zero fill characters. Allowing applications to set the field width enables them to agree on the number of digits to be printed and scanned in the ISO 8601:2019 standard expanded representation of a year (for %F, %G, and %Y) or all but the last two digits of the year (for %C). This is based on a feature in some versions of GNU libc's strftime(). The GNU version allows specifying space, zero, or no-fill characters in strftime() format strings, but does not allow any flags to be specified in strptime() format strings. These implementations also allow these flags to be specified for any numeric field. POSIX.1-2024 only requires the zero fill flag ('0') and only requires that it be recognized when processing %C, %F, %G, and %Y specifications when a minimum field width is also specified. The '0' flag is the only flag needed to produce and scan the ISO 8601:2019 standard year fields using the extended format forms. POSIX.1-2024 also allows applications to specify the same flag and field width specifiers to be used in both strftime() and strptime() format strings for symmetry. Systems may provide other flag characters and may accept flags in conjunction with conversion specifiers other than %C, %F, %G, and %Y; but portable applications cannot depend on such extensions.
POSIX.1-2024 now also allows the format specifications %+xC, %+xF, %+xG, and %+xY (where 'x' is a string of decimal digits used to specify printing and scanning of a string of 'x' decimal digits) with leading zero fill characters and a leading '+' sign character if the year being converted is more than four digits or a minimum field width is specified that allows room for more than four digits for the year. This allows date providers and consumers to agree on a specific number of digits to represent a year as required by the ISO 8601:2019 standard expanded representation formats. The expanded representation formats all require the year to begin with a leading '+' or '-' sign. (All of these specifiers can also provide a leading '-' sign for negative years. Since negative years and the year 0 don't fit well with the Gregorian or Julian calendars, the normal ranges of dates start with year 1. The ISO C standard allows tm_year to assume values corresponding to years before year 1, but the use of such years provided unspecified results.)
Some earlier version of this standard specified that applications wanting to use strptime() to scan dates and times printed by strftime() should provide non-digit characters between fields to separate years from months and days. It also supported %F to print and scan the ISO 8601:2019 standard extended format, complete representation date for years 1 through 9999 (i.e., YYYY-MM-DD). However, many applications were written to print (using strftime()) and scan (using strptime()) dates written using the basic format complete representation (four-digit years) and truncated representation (two-digit years) specified by the ISO 8601:2019 standard representation of dates and times which do not have any separation characters between fields. The ISO 8601:2019 standard also specifies basic format expanded representation where the creator and consumer of these fields agree beforehand to represent years as leading zero-filled strings of an agreed length of more than four digits to represent a year (again with no separation characters when year, month, and day are all displayed). Applications producing and consuming expanded representations are encouraged to use the '+' flag and an appropriate maximum field width to scan the year including the leading sign. Note that even without the '+' flag, years less than zero may be represented with a leading <hyphen-minus> for %F, %G, and %Y conversion specifications. Using negative years results in unspecified behavior.
If a format specification %+xF with the field width x greater than 11 is specified and the width is large enough to display the full year, the output string produced will match the ISO 8601:2019 standard subclause 4.1.2.4 expanded representation, extended format date representation for a specific day. (For years in the range [1,99999], %+12F is sufficient for an agreed five-digit year with a leading sign using the ISO 8601:2019 standard expanded representation, extended format for a specific day "±YYYYY-MM-DD".) Note also that years less than 0 may produce a leading <hyphen-minus> character ('-') when using %Y or %C whether or not the '0' or '+' flags are used.
The difference between the '0' flag and the '+' flag is whether the leading '+' character will be provided for years >9999 as required for the ISO 8601:2019 standard extended representation format containing a year. For example:
Year
Conversion Specification
strftime() Output
strptime() Scan Back
1970
%Y
1970
1970
1970
%+4Y
1970
1970
27
%Y
27 or 0027
27
270
%Y
270 or 0270
270
270
%+4Y
0270
270
17
%C%y
0017
17
270
%C%y
0270
270
12345
%Y
12345
1234*
12345
%+4Y
+12345
123*
12345
%05Y
12345
12345
270
%+5Y or %+3C%y
+0270
270
12345
%+5Y or %+3C%y
+12345
1234*
12345
%06Y or %04C%y
012345
12345
12345
%+6Y or %+4C%y
+12345
12345
123456
%08Y or %06C%y
00123456
123456
123456
%+8Y or %+6C%y
+0123456
123456
In the cases above marked with a * in the strptime() scan back field, the implied or specified number of characters scanned by strptime() was less than the number of characters output by strftime() using the same format; so the remaining digits of the year were dropped when the output date produced by strftime() was scanned back in by strptime().
None.
asctime, clock, ctime, difftime, futimens, getdate, gmtime, localtime, mktime, strptime, time, tzset, uselocale
XBD 7.3.5 LC_TIME, <time.h>
First released in Issue 3.
The description of %OV is changed to be consistent with %V and defines Monday as the first day of the week.
The description of %Oy is clarified.
Extensions beyond the ISO C standard are marked.
The Open Group Corrigendum U033/8 is applied. The %V conversion specifier is changed from "Otherwise, it is week 53 of the previous year, and the next week is week 1" to "Otherwise, it is the last week of the previous year, and the next week is week 1".
The following new requirements on POSIX implementations derive from alignment with the Single UNIX Specification:
- The %C, %D, %e, %h, %n, %r, %R, %t, and %T conversion specifiers are added.
- The modified conversion specifiers are added for consistency with the ISO POSIX-2 standard date utility.
The following changes are made for alignment with the ISO/IEC 9899:1999 standard:
- The strftime() prototype is updated.
- The DESCRIPTION is extensively revised.
- The %z conversion specifier is added.
An example is added.
IEEE Std 1003.1-2001/Cor 1-2002, item XSH/TC1/D6/60 is applied.
Austin Group Interpretation 1003.1-2001 #163 is applied.
The strftime_l() function is added from The Open Group Technical Standard, 2006, Extended API Set Part 4.
POSIX.1-2008, Technical Corrigendum 1, XSH/TC1-2008/0605 [283], XSH/TC1-2008/0606 [283], XSH/TC1-2008/0607 [193], and XSH/TC1-2008/0608 [193] are applied.
POSIX.1-2008, Technical Corrigendum 2, XSH/TC2-2008/0340 [584], XSH/TC2-2008/0341 [796], XSH/TC2-2008/0342 [584], and XSH/TC2-2008/0343 [584] are applied.
Austin Group Defects 169, 1386, and 1612 are applied, adding the s conversion and requiring errno to be unchanged on success and set on error.
Austin Group Defects 258 and 1166 are applied, adding the OB and Ob modified conversions.
Austin Group Defect 472 is applied, changing the description of the C conversion.
Austin Group Defect 739 is applied, changing the %F conversion to match the ISO C standard when no flag and no minimum field width are specified.
Austin Group Defect 1125 is applied, changing "Local timezone information is used" to "Local timezone information shall be set".
Austin Group Defect 1253 is applied, changing "daylight savings" to "daylight saving".
Austin Group Defect 1307 is applied, changing the r conversion in relation to locales that do not support the 12-hour clock format.
Austin Group Defects 1313 and 1354 are applied, changing text relating to the ISO 8601:2019 standard in the APPLICATION USAGE section.
Austin Group Defect 1462 is applied, changing the RATIONALE section.
Austin Group Defect 1533 is applied, adding tm_gmtoff and tm_zone to the tm structure.
Austin Group Defect 1562 is applied, clarifying that it is the application's responsibility to ensure that the format is a character string, beginning and ending in its initial shift state, if any.
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