github.com/afumu/libc@v0.0.6/musl/src/time/__year_to_secs.c (about)

     1  long long __year_to_secs(long long year, int *is_leap)
     2  {
     3  	if (year-2ULL <= 136) {
     4  		int y = year;
     5  		int leaps = (y-68)>>2;
     6  		if (!((y-68)&3)) {
     7  			leaps--;
     8  			if (is_leap) *is_leap = 1;
     9  		} else if (is_leap) *is_leap = 0;
    10  		return 31536000*(y-70) + 86400*leaps;
    11  	}
    12  
    13  	int cycles, centuries, leaps, rem;
    14  
    15  	if (!is_leap) is_leap = &(int){0};
    16  	cycles = (year-100) / 400;
    17  	rem = (year-100) % 400;
    18  	if (rem < 0) {
    19  		cycles--;
    20  		rem += 400;
    21  	}
    22  	if (!rem) {
    23  		*is_leap = 1;
    24  		centuries = 0;
    25  		leaps = 0;
    26  	} else {
    27  		if (rem >= 200) {
    28  			if (rem >= 300) centuries = 3, rem -= 300;
    29  			else centuries = 2, rem -= 200;
    30  		} else {
    31  			if (rem >= 100) centuries = 1, rem -= 100;
    32  			else centuries = 0;
    33  		}
    34  		if (!rem) {
    35  			*is_leap = 0;
    36  			leaps = 0;
    37  		} else {
    38  			leaps = rem / 4U;
    39  			rem %= 4U;
    40  			*is_leap = !rem;
    41  		}
    42  	}
    43  
    44  	leaps += 97*cycles + 24*centuries - *is_leap;
    45  
    46  	return (year-100) * 31536000LL + leaps * 86400LL + 946684800 + 86400;
    47  }