Pure moon-phase math you can test off the watch. It counts from a known new moon using the average length of a moon cycle, so it needs nothing but a UTC timestamp. More...

#include <time.h>
#include <stdbool.h>
#include <stdint.h>

Macros
#define	MOON_SYNODIC_SEC 2551443
	Average length of one moon cycle in seconds (about 29.53 days).
#define	MOON_EPOCH_UTC 947182440
	A known new moon to count from: 2000-01-06 18:14 UTC.

Functions
int32_t	moon_age_sec (time_t utc)
	How far into the current moon cycle we are, in seconds.
int	moon_glyph_index (time_t utc, int count)
	Picks one of `count` evenly spaced phase icons. It wraps back round at the end.
int	moon_illumination_pct (time_t utc)
	How lit up the moon looks, from 0 to 100. This is a rough straight-line guess, so it reads 0 at new moon and 100 at full.
const char *	moon_phase_name (time_t utc)
	Short name for the current phase (one of the eight main phases).
int	moon_days_to_phase (time_t utc, bool to_full)
	How many days until the next full or new moon, rounded to the nearest whole day.

Detailed Description

Pure moon-phase math you can test off the watch. It counts from a known new moon using the average length of a moon cycle, so it needs nothing but a UTC timestamp.

Macros

Functions

Detailed Description