POSIX does not require strptime() to derive tm_mon/tm_mday from %j, and musl
does not, so ordinal ISO 8601 dates (e.g. "2001-34") were mis-parsed on musl.

Bug: https://bugs.gentoo.org/922481
--- a/src/unix.c
+++ b/src/unix.c
@@ -22,8 +22,23 @@
         return IE_SUCCESS;
 
     offset = strptime((char*)string, "%Y-%j", time);
-    if (offset && *offset == '\0')
+    if (offset && *offset == '\0') {
+        /* POSIX does not require strptime() to derive tm_mon and tm_mday
+         * from %j, and musl does not (it only sets tm_yday). */
+        static const int days_before_month[12] =
+            {0, 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334};
+        const int year = time->tm_year + 1900;
+        const int leap =
+            ((year % 4 == 0) && (year % 100 != 0)) || (year % 400 == 0);
+        int mon = 11;
+        while ((mon > 0) &&
+            (time->tm_yday < days_before_month[mon] + ((mon >= 2) ? leap : 0)))
+            mon--;
+        time->tm_mon = mon;
+        time->tm_mday = time->tm_yday - days_before_month[mon]
+            - ((mon >= 2) ? leap : 0) + 1;
         return IE_SUCCESS;
+    }
 
     return IE_NOTSUP;
 }