#include <stdio.h>
int isrun(int year)
{
	if (year % 4 == 0 && year % 100 != 0 || year % 400 == 0)
		return 0;
	return 1;
}
int monthdays(int year, int month)
{
	int monthdays[12] = { 31,28,31,30,31,30,31,31,30,31,30,31 };
	if (month == 2 && isrun(year))
		return 29;
	else
		return monthdays[month - 1];
}
void a(long long totalsec)
{
	const long long daysec = 86400;
	const int hoursec = 3600;
	const int minsec = 60;
	int totalday = totalsec / daysec;
	int leftsec = totalsec % daysec;
	int hour = leftsec / hoursec;
	leftsec = leftsec % hoursec;
	int min = leftsec / minsec;
	int sec = leftsec % minsec;
	int year = 1970;
	while (1)
	{
		int yearday = isrun(year) ?
			366 : 365;
		if (totalday < yearday)
			break;
		totalday = totalday - yearday;
		year++;
	}
	int month = 1;
	while (1)
	{
		int monthday = monthdays(year, month);
		if (totalday < monthday)
			break;
		totalday = totalday - monthday;
		month++;
	}
	int day = totalday + 1;
	printf("%04d:%02d:%02d:%02d:%02d:%02d:", year, month, day, hour, min, sec);
}
int main()
{
	long long sec;
	printf("距离1970年1月1日零点的秒数: ");
	scanf_s("%lld", &sec);
	a(sec);
	return 0;

}

感觉有点复杂，在写几遍消化消化