Simplify floor division function.

src/mtime_calendarGregorian.c

@@ -11,6 +11,7 @@
 
 #include <stdint.h>
 #include <stddef.h>
+#include <stdlib.h>
 
 #include "mtime_calendarGregorian.h"
 #include "mtime_julianDay.h"
@@ -20,22 +21,13 @@
 static const int msn[12] =
   { 0, 31, 61, 92, 122, 153, 184, 214, 245, 275, 306, 337 };
 
-/* Internal function. */
+/* Compute floor division */
 static
 int64_t
 ifloor(int64_t ir1, int64_t ir2)
 {
-  int64_t r;
-  int64_t ifl;
-
-  r = (ir1 % ir2);
-  ifl = ir1 / ir2;
-
-  if (r < 0)
-    {
-      ifl = ifl - 1;
-    }
-  return ifl;
+  lldiv_t r = lldiv(ir1, ir2);
+  return r.quot - (r.rem < 0);
 }
 
 /**