<time.h>

Include the standard header <time.h> to declare several functions that help you manipulate times. The diagram summarizes the functions and the object types that they convert between.

The functions share two static-duration objects that hold values computed by the functions:

• a time string of type array of char
• a time structure of type struct tm

A call to one of these functions can alter the value that was stored earlier in a static-duration object by another of these functions.

    /* MACROS */
#define CLOCKS_PER_SEC <integer constant expression > 0>
#define NULL <either 0, 0L, or (void *)0> [0 in C++]

    /* TYPES */
typedef a-type clock_t;
typedef ui-type size_t;
typedef a-type time_t;
struct tm;

    /* FUNCTIONS */
char *asctime(const struct tm *tptr);
clock_t clock(void);
char *ctime(const time_t *tod);
double difftime(time_t t1, time_t t0);
struct tm *gmtime(const time_t *tod);
struct tm *localtime(const time_t *tod);
time_t mktime(struct tm *tptr);
size_t strftime(char *s, size_t n,
    const char *format, const struct tm *tptr);
time_t time(time_t *tod);

asctime

char *asctime(const struct tm *tptr);

The function stores in the static-duration time string a 26-character English-language representation of the time encoded in *tptr. It returns the address of the static-duration time string. The text representation takes the form:

Sun Dec  2 06:55:15 1979\n\0

clock

clock_t clock(void);

The function returns the number of clock ticks of elapsed processor time, counting from a time related to program startup, or it returns -1 if the target environment cannot measure elapsed processor time.

CLOCKS_PER_SEC

#define CLOCKS_PER_SEC <integer constant expression > 0>

The macro yields the number of clock ticks, returned by clock, in one second.

clock_t

typedef a-type clock_t;

The type is the arithmetic type a-type of an object that you declare to hold the value returned by clock, representing elapsed processor time.

ctime

char *ctime(const time_t *tod);

The function converts the calendar time in *tod to a text representation of the local time in the static-duration time string. It returns the address of that string. It is equivalent to asctime(localtime(tod)).

difftime

double difftime(time_t t1, time_t t0);

The function returns the difference t1 - t0, in seconds, between the calendar time t0 and the calendar time t1.

gmtime

struct tm *gmtime(const time_t *tod);

The function stores in the static-duration time structure an encoding of the calendar time in *tod, expressed as Coordinated Universal Time, or UTC [sic]. (UTC was formerly Greenwich Mean Time, or GMT). It returns the address of that structure, or [added with C99] a null pointer if it cannot generate the encoding.

localtime

struct tm *localtime(const time_t *tod);

The function stores in the static-duration time structure an encoding of the calendar time in *tod, expressed as local time. It returns the address of that structure, or [added with C99] a null pointer if it cannot generate the encoding.

mktime

time_t mktime(struct tm *tptr);

The function alters the values stored in *tptr to represent an equivalent encoded local time, but with the values of all members within their normal ranges. It then determines the values tptr->wday and tptr->yday from the values of the other members. It returns the calendar time equivalent to the encoded time, or it returns a value of -1 if the calendar time cannot be represented.

NULL

#define NULL <either 0, 0L, or (void *)0> [0 in C++]

The macro yields a null pointer constant that is usable as an address constant expression.

size_t

typedef ui-type size_t;

The type is the unsigned integer type ui-type of an object that you declare to store the result of the sizeof operator.

strftime

size_t strftime(char *s, size_t n,
    const char *format, const struct tm *tptr);

The function generates formatted text, under the control of the format format and the values stored in the time structure *tptr. It stores each generated character in successive locations of the array object of size n whose first element has the address s. The function then stores a null character in the next location of the array. It returns x, the number of characters generated, if x < n; otherwise, it returns zero, and the values stored in the array are indeterminate.

For each multibyte character other than % in the format, the function stores that multibyte character in the array object. Each occurrence of % followed by an optional qualifier and another character in the format is a conversion specifier. The optional qualifiers [added with C99] are:

• E, to represent times in terms of a locale-specific era (such as 1 BC instead of 0000).
• O, to represent numeric values with a set of locale-specific alternate digits (such as first instead of 1).

For each conversion specifier, the function stores a replacement character sequence.

The following table lists all conversion specifiers defined for strftime. The fields used in *tptr follow. Example replacement character sequences in parentheses follow each description. All examples are for the "C" locale, which ignores any optional qualifier, using the date and time Sunday, 2 December 1979 at 06:55:15 AM EST.

For a Sunday week of the year, week 1 begins with the first Sunday on or after 1 January. For a Monday week of the year, week 1 begins with the first Monday on or after 1 January. An ISO 8601 week of the year is the same as a Monday week of the year, with the following exceptions:

• If 1 January is a Tuesday, Wednesday, or Thursday, the week number is one greater. Moreover, days back to and including the immediately preceding Monday in the preceding year are included in week 1 of the current year.
• If 1 January is a Friday, Saturday, or Sunday, days up to but not including the immediately following Monday in the current year are included in the last week (52 or 53) of the preceding year.

Conversion specifications marked with a + in the table below are all added with C99:

SPECIFIER   FIELDS   DESCRIPTION (EXAMPLE)
    %a      tm_wday  abbreviated weekday name (Sun)
    %A      tm_wday  full weekday name (Sunday)
    %b      tm_mon   abbreviated month name (Dec)
    %B      tm_mon   full month name (December)
    %c       [all]   date and time (Sun Dec  2 06:55:15 1979)
    %Ec      [all]   + era-specific date and time
    %C      tm_year  + year/100 (19)
    %EC     tm_mday  + era specific era name
            tm_mon
            tm_year
    %d      tm_mday  day of the month (02)
    %D      tm_mday  + month/day/year from 01/01/00 (12/02/79)
            tm_mon
            tm_year
    %e      tm_mday  + day of the month, leading space for zero ( 2)
    %F      tm_mday  + year-month-day (1979-12-02)
            tm_mon
            tm_year
    %g      tm_wday  + year for ISO 8601 week, from 00 (79)
            tm_yday
            tm_year
    %G      tm_wday  + year for ISO 8601 week, from 0000 (1979)
            tm_yday
            tm_year
    %h      tm_mon   + same as %b (Dec)
    %H      tm_hour  hour of the 24-hour day, from 00 (06)
    %I      tm_hour  hour of the 12-hour day, from 01 (06)
    %j      tm_yday  day of the year, from 001 (336)
    %m      tm_mon   month of the year, from 01 (12)
    %M      tm_min   minutes after the hour (55)
    %n               + newline character \n
    %p      tm_hour  AM/PM indicator (AM)
    %r      tm_sec   + 12-hour time, from 01:00:00 AM (06:55:15 AM)
            tm_min
            tm_hour
    %Er     tm_sec   + era-specific date and 12-hour time
            tm_min
            tm_hour
            tm_mday
            tm_mon
            tm_year
    %R      tm_min   + hour:minute, from 01:00 (06:55)
            tm_hour
    %S      tm_sec   seconds after the minute (15)
    %t               + horizontal tab character \t
    %T      tm_sec   + 24-hour time, from 00:00:00 (06:55:15)
            tm_min
            tm_hour
    %u      tm_wday  + ISO 8601 day of the week, to 7 for Sunday (7)
    %U      tm_wday  Sunday week of the year, from 00 (48)
            tm_yday
    %V      tm_wday  + ISO 8601 week of the year, from 01 (48)
            tm_yday
            tm_year
    %w      tm_wday  day of the week, from 0 for Sunday (0)
    %W      tm_wday  Monday week of the year, from 00 (48)
            tm_yday
    %x       [all]   date (02/12/79)
    %Ex      [all]   + era-specific date
    %X       [all]   time, from 00:00:00 (06:55:15)
    %EX      [all]   + era-specific time
    %y      tm_year  year of the century, from 00 (79)
    %Ey     tm_mday  + year of the era
            tm_mon
            tm_year
    %Y      tm_year  year (1979)
    %EY     tm_mday  + era-specific era name and year of the era
            tm_mon
            tm_year
    %z      tm_isdst + time zone (hours east*100 + minutes), if any (-0500)
    %Z      tm_isdst time zone name, if any (EST)
    %%               percent character %

The current locale category LC_TIME can affect these replacement character sequences.

time

time_t time(time_t *tod);

If tod is not a null pointer, the function stores the current calendar time in *tod. The function returns the current calendar time, if the target environment can determine it; otherwise, it returns -1.

time_t

typedef a-type time_t;

The type is the arithmetic type a-type of an object that you declare to hold the value returned by time. The value represents calendar time.

tm

struct tm {
    int tm_sec;        seconds after the minute (from 0)
    int tm_min;        minutes after the hour (from 0)
    int tm_hour;       hour of the day (from 0)
    int tm_mday;       day of the month (from 1)
    int tm_mon;        month of the year (from 0)
    int tm_year;       years since 1900 (from 0)
    int tm_wday;       days since Sunday (from 0)
    int tm_yday;       day of the year (from 0)
    int tm_isdst;      Daylight Saving Time flag
    };

struct tm contains members that describe various properties of the calendar time. The members shown above can occur in any order, interspersed with additional members. The comment following each member briefly describes its meaning.

The member tm_isdst contains:

• a positive value if Daylight Saving Time is in effect
• zero if Daylight Saving Time is not in effect
• a negative value if the status of Daylight Saving Time is not known (so the target environment should attempt to determine its status)

See also the Table of Contents and the Index.

Copyright © 1989-2002 by P.J. Plauger and Jim Brodie. All rights reserved.