<time.h>: Time

Data Structures
struct	tm
struct	week_date

Macros
#define	ONE_HOUR   3600
#define	ONE_DEGREE   3600
#define	ONE_DAY   86400
#define	UNIX_OFFSET   946684800
#define	NTP_OFFSET   3155673600

Typedefs
typedef uint32_t	time_t

Enumerations
enum	_WEEK_DAYS_ {   SUNDAY, MONDAY, TUESDAY, WEDNESDAY,   THURSDAY, FRIDAY, SATURDAY }
enum	_MONTHS_ {   JANUARY, FEBRUARY, MARCH, APRIL,   MAY, JUNE, JULY, AUGUST,   SEPTEMBER, OCTOBER, NOVEMBER, DECEMBER }

Functions
time_t	time (time_t *timer)
int32_t	difftime (time_t time1, time_t time0)
time_t	mktime (struct tm *timeptr)
time_t	mk_gmtime (const struct tm *timeptr)
struct tm *	gmtime (const time_t *timer)
void	gmtime_r (const time_t *timer, struct tm *timeptr)
struct tm *	localtime (const time_t *timer)
void	localtime_r (const time_t *timer, struct tm *timeptr)
char *	asctime (const struct tm *timeptr)
void	asctime_r (const struct tm *timeptr, char *buf)
char *	ctime (const time_t *timer)
void	ctime_r (const time_t *timer, char *buf)
char *	isotime (const struct tm *tmptr)
void	isotime_r (const struct tm *, char *)
size_t	strftime (char *s, size_t maxsize, const char *format, const struct tm *timeptr)
void	set_dst (int(*)(const time_t *, int32_t *))
void	set_zone (int32_t)
void	set_system_time (time_t timestamp)
void	system_tick (void)
uint8_t	is_leap_year (int16_t year)
uint8_t	month_length (int16_t year, uint8_t month)
uint8_t	week_of_year (const struct tm *timeptr, uint8_t start)
uint8_t	week_of_month (const struct tm *timeptr, uint8_t start)
struct week_date *	iso_week_date (int year, int yday)
void	iso_week_date_r (int year, int yday, struct week_date *)
uint32_t	fatfs_time (const struct tm *timeptr)
void	set_position (int32_t latitude, int32_t longitude)
int16_t	equation_of_time (const time_t *timer)
int32_t	daylight_seconds (const time_t *timer)
time_t	solar_noon (const time_t *timer)
time_t	sun_rise (const time_t *timer)
time_t	sun_set (const time_t *timer)
double	solar_declination (const time_t *timer)
int8_t	moon_phase (const time_t *timer)
unsigned long	gm_sidereal (const time_t *timer)
unsigned long	lm_sidereal (const time_t *timer)

Detailed Description

#include <time.h> 

Introduction to the Time functions

This file declares the time functions implemented in avr-libc.

The implementation aspires to conform with ISO/IEC 9899 (C90). However, due to limitations of the target processor and the nature of its development environment, a practical implementation must of necessity deviate from the standard.

Section 7.23.2.1 clock() The type clock_t, the macro CLOCKS_PER_SEC, and the function clock() are not implemented. We consider these items belong to operating system code, or to application code when no operating system is present.

Section 7.23.2.3 mktime() The standard specifies that mktime() should return (time_t) -1, if the time cannot be represented. This implementation always returns a 'best effort' representation.

Section 7.23.2.4 time() The standard specifies that time() should return (time_t) -1, if the time is not available. Since the application must initialize the time system, this functionality is not implemented.

Section 7.23.2.2, difftime() Due to the lack of a 64 bit double, the function difftime() returns a long integer. In most cases this change will be invisible to the user, handled automatically by the compiler.

Section 7.23.1.4 struct tm Per the standard, struct tm->tm_isdst is greater than zero when Daylight Saving time is in effect. This implementation further specifies that, when positive, the value of tm_isdst represents the amount time is advanced during Daylight Saving time.

Section 7.23.3.5 strftime() Only the 'C' locale is supported, therefore the modifiers 'E' and 'O' are ignored. The 'Z' conversion is also ignored, due to the lack of time zone name.

In addition to the above departures from the standard, there are some behaviors which are different from what is often expected, though allowed under the standard.

There is no 'platform standard' method to obtain the current time, time zone, or daylight savings 'rules' in the AVR environment. Therefore the application must initialize the time system with this information. The functions set_zone(), set_dst(), and set_system_time() are provided for initialization. Once initialized, system time is maintained by calling the function system_tick() at one second intervals.

Though not specified in the standard, it is often expected that time_t is a signed integer representing an offset in seconds from Midnight Jan 1 1970... i.e. 'Unix time'. This implementation uses an unsigned 32 bit integer offset from Midnight Jan 1 2000. The use of this 'epoch' helps to simplify the conversion functions, while the 32 bit value allows time to be properly represented until Tue Feb 7 06:28:15 2136 UTC. The macros UNIX_OFFSET and NTP_OFFSET are defined to assist in converting to and from Unix and NTP time stamps.

Unlike desktop counterparts, it is impractical to implement or maintain the 'zoneinfo' database. Therefore no attempt is made to account for time zone, daylight saving, or leap seconds in past dates. All calculations are made according to the currently configured time zone and daylight saving 'rule'.

In addition to C standard functions, re-entrant versions of ctime(), asctime(), gmtime() and localtime() are provided which, in addition to being re-entrant, have the property of claiming less permanent storage in RAM. An additional time conversion, isotime() and its re-entrant version, uses far less storage than either ctime() or asctime().

Along with the usual smattering of utility functions, such as is_leap_year(), this library includes a set of functions related the sun and moon, as well as sidereal time functions.

Macro Definition Documentation

NTP_OFFSET

#define NTP_OFFSET   3155673600

Difference between the Y2K and the NTP epochs, in seconds. To convert a Y2K timestamp to NTP...

unsigned long ntp;
time_t y2k;

y2k = time(NULL);
ntp = y2k + NTP_OFFSET;

ONE_DAY

#define ONE_DAY   86400

One day, expressed in seconds

ONE_DEGREE

#define ONE_DEGREE   3600

Angular degree, expressed in arc seconds

ONE_HOUR

#define ONE_HOUR   3600

One hour, expressed in seconds

UNIX_OFFSET

#define UNIX_OFFSET   946684800

Difference between the Y2K and the UNIX epochs, in seconds. To convert a Y2K timestamp to UNIX...

long unix;
time_t y2k;

y2k = time(NULL);
unix = y2k + UNIX_OFFSET;

Typedef Documentation

time_t

typedef uint32_t time_t

time_t represents seconds elapsed from Midnight, Jan 1 2000 UTC (the Y2K 'epoch'). Its range allows this implementation to represent time up to Tue Feb 7 06:28:15 2136 UTC.

Enumeration Type Documentation

_MONTHS_

enum _MONTHS_

Enumerated labels for the months.

_WEEK_DAYS_

enum _WEEK_DAYS_

Enumerated labels for the days of the week.

Function Documentation

asctime()

char* asctime

(

const struct tm *

timeptr

)

The asctime function converts the broken-down time of timeptr, into an ascii string in the form

Sun Mar 23 01:03:52 2013

asctime_r()

void asctime_r	(	const struct tm *	timeptr,
		char *	buf
	)

Re entrant version of asctime().

ctime()

char* ctime

(

const time_t *

timer

)

The ctime function is equivalent to asctime(localtime(timer))

ctime_r()

void ctime_r	(	const time_t *	timer,
		char *	buf
	)

Re entrant version of ctime().

daylight_seconds()

int32_t daylight_seconds

(

const time_t *

timer

)

Computes the amount of time the sun is above the horizon, at the location of the observer.

NOTE: At observer locations inside a polar circle, this value can be zero during the winter, and can exceed ONE_DAY during the summer.

The returned value is in seconds.

difftime()

int32_t difftime	(	time_t	time1,
		time_t	time0
	)

The difftime function returns the difference between two binary time stamps, time1 - time0.

equation_of_time()

int16_t equation_of_time

(

const time_t *

timer

)

Computes the difference between apparent solar time and mean solar time. The returned value is in seconds.

fatfs_time()

uint32_t fatfs_time

(

const struct tm *

timeptr

)

Convert a Y2K time stamp into a FAT file system time stamp.

gm_sidereal()

unsigned long gm_sidereal

(

const time_t *

timer

)

Returns Greenwich Mean Sidereal Time, as seconds into the sidereal day. The returned value will range from 0 through 86399 seconds.

gmtime()

struct tm* gmtime

(

const time_t *

timer

)

The gmtime function converts the time stamp pointed to by timer into broken-down time, expressed as UTC.

gmtime_r()

void gmtime_r	(	const time_t *	timer,
		struct tm *	timeptr
	)

Re entrant version of gmtime().

is_leap_year()

uint8_t is_leap_year

(

int16_t

year

)

Return 1 if year is a leap year, zero if it is not.

iso_week_date()

struct week_date* iso_week_date	(	int	year,
		int	yday
	)

Return a week_date structure with the ISO_8601 week based date corresponding to the given year and day of year. See http://en.wikipedia.org/wiki/ISO_week_date for more information.

iso_week_date_r()

void iso_week_date_r	(	int	year,
		int	yday,
		struct week_date *
	)

Re-entrant version of iso-week_date.

isotime()

char* isotime

(

const struct tm *

tmptr

)

The isotime function constructs an ascii string in the form

2013-03-23 01:03:52

isotime_r()

void isotime_r	(	const struct tm *	,
		char *
	)

Re entrant version of isotime()

lm_sidereal()

unsigned long lm_sidereal

(

const time_t *

timer

)

Returns Local Mean Sidereal Time, as seconds into the sidereal day. The returned value will range from 0 through 86399 seconds.

localtime()

struct tm* localtime

(

const time_t *

timer

)

The localtime function converts the time stamp pointed to by timer into broken-down time, expressed as Local time.

localtime_r()

void localtime_r	(	const time_t *	timer,
		struct tm *	timeptr
	)

Re entrant version of localtime().

mk_gmtime()

time_t mk_gmtime

(

const struct tm *

timeptr

)

This function 'compiles' the elements of a broken-down time structure, returning a binary time stamp. The elements of timeptr are interpreted as representing UTC.

The original values of the tm_wday and tm_yday elements of the structure are ignored, and the original values of the other elements are not restricted to the ranges stated for struct tm.

Unlike mktime(), this function DOES NOT modify the elements of timeptr.

mktime()

time_t mktime

(

struct tm *

timeptr

)

This function 'compiles' the elements of a broken-down time structure, returning a binary time stamp. The elements of timeptr are interpreted as representing Local Time.

The original values of the tm_wday and tm_yday elements of the structure are ignored, and the original values of the other elements are not restricted to the ranges stated for struct tm.

On successful completion, the values of all elements of timeptr are set to the appropriate range.

month_length()

uint8_t month_length	(	int16_t	year,
		uint8_t	month
	)

Return the length of month, given the year and month, where month is in the range 1 to 12.

moon_phase()

int8_t moon_phase

(

const time_t *

timer

)

Returns an approximation to the phase of the moon. The sign of the returned value indicates a waning or waxing phase. The magnitude of the returned value indicates the percentage illumination.

set_dst()

void set_dst

(

int(*)(const time_t *, int32_t *)

)

Specify the Daylight Saving function.

The Daylight Saving function should examine its parameters to determine whether Daylight Saving is in effect, and return a value appropriate for tm_isdst.

Working examples for the USA and the EU are available..

#include <util/eu_dst.h>

for the European Union, and

#include <util/usa_dst.h>

for the United States

If a Daylight Saving function is not specified, the system will ignore Daylight Saving.

set_position()

void set_position	(	int32_t	latitude,
		int32_t	longitude
	)

Set the geographic coordinates of the 'observer', for use with several of the following functions. Parameters are passed as seconds of North Latitude, and seconds of East Longitude.

For New York City...

set_position( 40.7142 * ONE_DEGREE, -74.0064 * ONE_DEGREE); 

set_system_time()

void set_system_time

(

time_t

timestamp

)

Initialize the system time. Examples are...

From a Clock / Calendar type RTC:

struct tm rtc_time;

read_rtc(&rtc_time);
rtc_time.tm_isdst = 0;
set_system_time( mktime(&rtc_time) );

From a Network Time Protocol time stamp:

set_system_time(ntp_timestamp - NTP_OFFSET);

From a UNIX time stamp:

set_system_time(unix_timestamp - UNIX_OFFSET);

set_zone()

void set_zone

(

int32_t

)

Set the 'time zone'. The parameter is given in seconds East of the Prime Meridian. Example for New York City:

set_zone(-5 * ONE_HOUR);

If the time zone is not set, the time system will operate in UTC only.

solar_declination()

double solar_declination

(

const time_t *

timer

)

Returns the declination of the sun in radians.

solar_noon()

time_t solar_noon

(

const time_t *

timer

)

Computes the time of solar noon, at the location of the observer.

strftime()

size_t strftime	(	char *	s,
		size_t	maxsize,
		const char *	format,
		const struct tm *	timeptr
	)

A complete description of strftime() is beyond the pale of this document. Refer to ISO/IEC document 9899 for details.

All conversions are made using the 'C Locale', ignoring the E or O modifiers. Due to the lack of a time zone 'name', the 'Z' conversion is also ignored.

sun_rise()

time_t sun_rise

(

const time_t *

timer

)

Return the time of sunrise, at the location of the observer. See the note about daylight_seconds().

sun_set()

time_t sun_set

(

const time_t *

timer

)

Return the time of sunset, at the location of the observer. See the note about daylight_seconds().

system_tick()

void system_tick

(

void

)

Maintain the system time by calling this function at a rate of 1 Hertz.

It is anticipated that this function will typically be called from within an Interrupt Service Routine, (though that is not required). It therefore includes code which makes it simple to use from within a 'Naked' ISR, avoiding the cost of saving and restoring all the cpu registers.

Such an ISR may resemble the following example...

ISR(RTC_OVF_vect, ISR_NAKED)
{
    system_tick();
    reti();
}

time()

time_t time

(

time_t *

timer

)

The time function returns the systems current time stamp. If timer is not a null pointer, the return value is also assigned to the object it points to.

week_of_month()

uint8_t week_of_month	(	const struct tm *	timeptr,
		uint8_t	start
	)

Return the calendar week of month, where the first week is considered to begin on the day of week specified by 'start'. The returned value may range from zero to 5.

week_of_year()

uint8_t week_of_year	(	const struct tm *	timeptr,
		uint8_t	start
	)

Return the calendar week of year, where week 1 is considered to begin on the day of week specified by 'start'. The returned value may range from zero to 52.