libcalendars is an experimental C library to provide arithmentic for several calendars.

Motivation

Why implement yet another calendaring library? Well, there are plenty of excellent implementations out there, though it seems there is no free, GPL-compliant, C implementation. Besides there is no precise implementation of non-gregorian calendars, most importantly Solar Hijri and Islamic Civil calendars.

API

Almost all algorithms in libcalendar are implemented using Julian Day calculations. You can convert any date on supported calendars to JDN and vice versa. For example:

uint32_t jdn = 0;
sh_to_jdn(&jdn, 1392, 04, 15);
printf("Julian Day for 1392/04/15 AP is: %l\n", jdn);

Which prints:

1 $ Julian Day for 1392/04/15 AP is: 2456480

You can also use non-jdn broken-down date API. For example you can check if a year in Solar Hijri calendar is leap or not:

if(sh_is_leap(1395)) /* returns 0 for regular years and 1 for leap years */
    printf("Yep\n");
printf("Number of days in 1395: %d\n", sh_days_in_year(1395));

Conversion algorithms for dates in all calendars to/from Gregorian calendar is supported through JDN to date conversions:

int16_t y;
uint8_t m;
uint16_t d;
sh_to_gr(1369, 06, 20, &y, &m, &d);
printf("1369/06/20 AP is %04d-%02d-%02d\n", y, m, d);
gr_to_sh(2017, 09, 11, &y, &m, &d); 
printf("2017-09-11 is %04d-%02d-%02d AP\n", y, m, d);

which will print:

1 1369/06/20 AP is 2017-09-11

2 2017-09-11 is 1369/06/18 AP

Function Naming

Each calendar is identified by a two character abbriviation, which will be used either as a prefix or a postfix to all function names. Normally a supported calendar will have following functions, which cl is calendar abbreviation.

cl_is_leap
cl_days_in_month
cl_days_in_year
cl_month_in_year
cl_is_valid
cl_to_jdn
jdn_to_cl
cl_to_gr
gr_to_cl

Supported Calendars

Calendar Name	Alternative Names	Abbriviation
Gregorian		`gr`
Julian		`ju`
Milankovic	Revised Julian	`ml`
Jewish	Hebrew	`jw`
Solar Hijri	Shamsi Persian Iranian	`sh`