rtc: Fix rtc_calc_weekday

This function appeared previously unused (called only from rtc_display, also unused),
but it returned an incorrect weekday.  Change the algorithm to use Zeller's Rule, a
well-known algorithm for calculuating weekdays.

Change-Id: Ibce6822942f8d9d9f39c2b6065cd785dca9e8e09
Signed-off-by: Tim Wawrzynczak <twawrzynczak@chromium.org>
Reviewed-on: https://review.coreboot.org/c/31557
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Julius Werner <jwerner@chromium.org>

src/lib/rtc.c

@@ -27,50 +27,34 @@
 #define DAYS_IN_YEAR(a)		(LEAP_YEAR(a) ? 366 : 365)
 #define DAYS_IN_MONTH(a)	(month_days[(a) - 1])
 
-static const int month_offset[] = {
-	0,  31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334
-};
-
 static const char *const weekdays[] = {
-	"Sun",  "Mon",  "Tues",  "Wednes",  "Thurs",  "Fri",  "Satur",
+	"Sun", "Mon", "Tues", "Wednes", "Thurs", "Fri", "Satur"
 };
 
-static int leaps_to_year(int year)
-{
-	return year / 4 - year / 100 + year / 400;
-}
-
-/* This only works for the Gregorian calendar after Jan 1 1971. */
+/* Zeller's rule */
 static int rtc_calc_weekday(struct rtc_time *tm)
 {
-	int leaps_to_date;
-	int day;
-
 	if (tm->year < 1971)
 		return -1;
 
-	day = 4; /* Jan 1 1970 was a Thursday. */
-
-	/* Number of leap corrections to apply up to end of last year */
-	leaps_to_date = leaps_to_year(tm->year - 1) - leaps_to_year(1970);
+	/* In Zeller's rule, January and February are treated as if they
+	   are months 13 and 14 of the previous year (March is still month 3) */
+	const int zyear = ((tm->mon < 3) ? tm->year - 1 : tm->year);
+	const int q = tm->mday;
+	const int m = (tm->mon < 3) ? tm->mon + 12 : tm->mon;
+	const int K = zyear % 100;
+	const int J = zyear / 100;
 
 	/*
-	 * This year is a leap year if it is divisible by 4 except when it is
-	 * divisible by 100 unless it is divisible by 400
-	 *
-	 * e.g. 1904 was a leap year,  1900 was not,  1996 is, and 2000 is.
+	 * Because of the way the modulo operator works with negative numbers,
+	 * the traditional formulation of Zeller's rule must be modified
+	 * slightly to make the numerator positive (i.e., add 5J instead of
+	 * subtracting 2J).  Also subtract 1 so that Sunday is day 0.
 	 */
-	if ((tm->year % 4) &&
-	    ((tm->year % 100 != 0) || (tm->year % 400 == 0)) &&
-	    (tm->mon > 2)) {
-		/* We are past Feb. 29 in a leap year */
-		day++;
-	}
-
-	day += (tm->year - 1970) * 365 + leaps_to_date +
-		month_offset[tm->mon-1] + tm->mday;
-	tm->wday = day % 7;
+	const int h = (q + (13 * (m + 1)) / 5
+		       + K + (K / 4) + (J / 4) + (5 * J) - 1) % 7;
 
+	tm->wday = h;
 	return 0;
 }
 
@@ -147,7 +131,7 @@ unsigned long rtc_mktime(const struct rtc_time *tm)
 
 void rtc_display(const struct rtc_time *tm)
 {
-	printk(BIOS_INFO,  "Date: %4d-%02d-%02d (%sday)  Time: %2d:%02d:%02d\n",
+	printk(BIOS_INFO, "Date: %5d-%02d-%02d (%sday)  Time: %2d:%02d:%02d\n",
 	       tm->year,  tm->mon,  tm->mday,
 	       (tm->wday < 0 || tm->wday > 6) ? "unknown " : weekdays[tm->wday],
 	       tm->hour,  tm->min,  tm->sec);