diff --git a/loslib.c b/loslib.c
index 4623ad5e..3f605028 100644
--- a/loslib.c
+++ b/loslib.c
@@ -273,7 +273,7 @@ static int getfield (lua_State *L, const char *key, int d, int delta) {
 
 
 static const char *checkoption (lua_State *L, const char *conv,
-                                ptrdiff_t convlen, char *buff) {
+                                size_t convlen, char *buff) {
   const char *option = LUA_STRFTIMEOPTIONS;
   unsigned oplen = 1;  /* length of options being checked */
   for (; *option != '\0' && oplen <= convlen; option += oplen) {
@@ -333,7 +333,8 @@ static int os_date (lua_State *L) {
         size_t reslen;
         char *buff = luaL_prepbuffsize(&b, SIZETIMEFMT);
         s++;  /* skip '%' */
-        s = checkoption(L, s, se - s, cc + 1);  /* copy specifier to 'cc' */
+        /* copy specifier to 'cc' */
+        s = checkoption(L, s, ct_diff2sz(se - s), cc + 1);
         reslen = strftime(buff, SIZETIMEFMT, cc, stm);
         luaL_addsize(&b, reslen);
       }