date.c File Reference

Pure calendar math, host-testable off-device. More...

#include "clock/date.h"

Functions
static int	is_leap (int year)
	Whether a year is a leap year, by the usual rule.
static int	dec31_wday (int year)
	The weekday December 31st lands on for a year.
int	date_iso_weeks_in_year (int year)
	How many ISO 8601 weeks a year holds, either 52 or 53.
static int	raw_week (int yday, int wday)
	The week number worked out from the date alone, before the year boundary is taken into account.
int	date_iso_week (int year, int yday, int wday)
	The ISO 8601 week number, which always counts weeks from Monday.
int	date_iso_week_year (int year, int yday, int wday)
	The ISO 8601 year the week number belongs to, which is not always the calendar year.
int	date_day_of_year (int yday)
	Which day of the year it is, counting January 1st as day 1.
int	date_days_left_in_year (int year, int yday)
	How many days are left before the year ends, so December 31st reads 0.
int	date_days_in_month (int year, int mon0)
	How many days a month has, with February getting its leap day.
int	date_first_wday (int wday, int mday)
	The weekday the 1st of the month lands on, walked back from today's weekday.

Detailed Description

Pure calendar math, host-testable off-device.

Function Documentation

dec31_wday()

int dec31_wday ( int year )

static

The weekday December 31st lands on for a year.

Parameters

year	The full year (2026, not 126).

Returns

The weekday, 0 Sunday to 6 Saturday.

is_leap()

int is_leap ( int year )

static

Whether a year is a leap year, by the usual rule.

Every fourth year is one, except a century year, unless it also divides evenly by 400.

Parameters

year	The full year (2026, not 126).

Returns

True when the year has a leap day, false when it does not.

raw_week()

int raw_week ( int yday, int wday )

static

The week number worked out from the date alone, before the year boundary is taken into account.

A number below 1 means the day belongs to last year's final week. A number above the year's own total means it already belongs to next year's week 1. The ISO weekday used here runs Monday as 1 through Sunday as 7.

Parameters

yday	The day of the year counting January 1st as 0 (like tm_yday).
wday	The weekday, 0 Sunday to 6 Saturday.

Returns

The raw week number, which may fall outside 1 to the year's own week count.