rtc: Make rtc_time64_to_tm() support dates before 1970

 * rtc_time64_to_tm - converts time64_t to rtc_time.

 *

 * @time:	The number of seconds since 01-01-1970 00:00:00.

 *		(Must be positive.)

 *		Works for values since at least 1900

 * @tm:		Pointer to the struct rtc_time.

 */

void rtc_time64_to_tm(time64_t time, struct rtc_time *tm)

{

	unsigned int secs;

	int days;

	int days, secs;

	u64 u64tmp;

	u32 u32tmp, udays, century, day_of_century, year_of_century, year,

		day_of_year, month, day;

	bool is_Jan_or_Feb, is_leap_year;

	/* time must be positive */

	/*

	 * Get days and seconds while preserving the sign to

	 * handle negative time values (dates before 1970-01-01)

	 */

	days = div_s64_rem(time, 86400, &secs);

	/*

	 * We need 0 <= secs < 86400 which isn't given for negative

	 * values of time. Fixup accordingly.

	 */

	if (secs < 0) {

		days -= 1;

		secs += 86400;

	}

	/* day of the week, 1970-01-01 was a Thursday */

	tm->tm_wday = (days + 4) % 7;

	/* Ensure tm_wday is always positive */

	if (tm->tm_wday < 0)

		tm->tm_wday += 7;

	/*

	 * The following algorithm is, basically, Proposition 6.3 of Neri

	 * thus, is slightly different from [1].

	 */

	udays		= ((u32) days) + 719468;

	udays		= days + 719468;

	u32tmp		= 4 * udays + 3;

	century		= u32tmp / 146097;