1 /*-
2  * SPDX-License-Identifier: BSD-2-Clause
63  * Compute the Julian date from the number of days elapsed since
73 	 * Compute the year by starting with an approximation not smaller  in jdate()
81 		idt.y--;  in jdate()
84 	 * Set r to the days left in the year and compute the month by  in jdate()
88 	r = ndays - r;  in jdate()
89 	for (idt.m = 11; month1[idt.m] > r; idt.m--)  in jdate()
92 	/* Compute the days left in the month */  in jdate()
93 	idt.d = r - month1[idt.m];  in jdate()
109 		return (-1);  in ndaysj()
122 	return (idt->d + month1[idt->m] + idt->y * 365 + idt->y / 4);  in ndaysji()
126  * Compute the date according to the Gregorian calendar from the number of
128  * is older than 1582-10-05. This is the reverse of the function ndaysg().
133 	int const *montht;	/* month-table */  in gdate()
138 	 * Compute the year by starting with an approximation not smaller  in gdate()
146 		idt.y--;  in gdate()
149 	 * Set ndays to the number of days left and compute by linear  in gdate()
158 	ndays = ndays - r;  in gdate()
164 	for (idt.m = 11; montht[idt.m] > ndays; idt.m--)  in gdate()
167 	idt.d = ndays - montht[idt.m]; /* the rest is the day in month */  in gdate()
179  * assumed Gregorian if younger than 1582-10-04 and Julian otherwise. This
188 		return (-1);  in ndaysg()
199 	int     nd;		/* Number of days--return value */  in ndaysgi()
212 	 * 365d +.25d - .01d + .0025d = 365.2425d. But the tropical  in ndaysgi()
214 	 * again one day ahead of the earth. Sigh :-)  in ndaysgi()
218 	if ((nd = ndaysji(idt)) == -1)  in ndaysgi()
219 		return (-1);  in ndaysgi()
220 	if (idt->y >= 1600)  in ndaysgi()
221 		nd = (nd - 10 - (idt->y - 1600) / 100 + (idt->y - 1600) / 400);  in ndaysgi()
223 		nd -= 10;  in ndaysgi()
228  * Compute the week number from the number of days since March 1st year 0.
242 	for (*y = dt.y + 1; nd < (fw = firstweek(*y)); (*y)--)  in week()
244 	return ((nd - fw) / 7 + 1);  in week()
254 	idt.y = y - 1;   /* internal representation of y-1-1 */  in firstweek()
266 		return (nd - wd + 7);  in firstweek()
268 		return (nd - wd);  in firstweek()
275 	date dmondaygi = {1997, 8, 16}; /* Internal repr. of 1997-11-17 */  in weekday()
282 	/* return (nd - nmonday) modulo 7 which is the weekday */  in weekday()
283 	nd = (nd - nmonday) % 7;  in weekday()
299 	idt->d = dt->d - 1;  in date2idt()
300 	if (dt->m > 2) {  in date2idt()
301 		idt->m = dt->m - 3;  in date2idt()
302 		idt->y = dt->y;  in date2idt()
304 		idt->m = dt->m + 9;  in date2idt()
305 		idt->y = dt->y - 1;  in date2idt()
307 	if (idt->m < 0 || idt->m > 11 || idt->y < 0)  in date2idt()
318 	dt->d = idt->d + 1;  in idt2date()
319 	if (idt->m < 10) {  in idt2date()
320 		dt->m = idt->m + 3;  in idt2date()
321 		dt->y = idt->y;  in idt2date()
323 		dt->m = idt->m - 9;  in idt2date()
324 		dt->y = idt->y + 1;  in idt2date()
326 	if (dt->m < 1)  in idt2date()