Add a "fork" of musl as //fusl.

fusl/src/time/__year_to_secs.c

+long long __year_to_secs(long long year, int *is_leap)
+{
+        if (year-2ULL <= 136) {
+                int y = year;
+                int leaps = (y-68)>>2;
+                if (!((y-68)&3)) {
+                        leaps--;
+                        if (is_leap) *is_leap = 1;
+                } else if (is_leap) *is_leap = 0;
+                return 31536000*(y-70) + 86400*leaps;
+        }
+
+        int cycles, centuries, leaps, rem;
+
+        if (!is_leap) is_leap = &(int){0};
+        cycles = (year-100) / 400;
+        rem = (year-100) % 400;
+        if (rem < 0) {
+                cycles--;
+                rem += 400;
+        }
+        if (!rem) {
+                *is_leap = 1;
+                centuries = 0;
+                leaps = 0;
+        } else {
+                if (rem >= 200) {
+                        if (rem >= 300) centuries = 3, rem -= 300;
+                        else centuries = 2, rem -= 200;
+                } else {
+                        if (rem >= 100) centuries = 1, rem -= 100;
+                        else centuries = 0;
+                }
+                if (!rem) {
+                        *is_leap = 0;
+                        leaps = 0;
+                } else {
+                        leaps = rem / 4U;
+                        rem %= 4U;
+                        *is_leap = !rem;
+                }
+        }
+
+        leaps += 97*cycles + 24*centuries - *is_leap;
+
+        return (year-100) * 31536000LL + leaps * 86400LL + 946684800 + 86400;
+}