diff --git a/external/lua-5.1.3/src/loslib.c b/external/lua-5.1.3/src/loslib.c
index e91c56c10..20af17e86 100644
--- a/external/lua-5.1.3/src/loslib.c
+++ b/external/lua-5.1.3/src/loslib.c
@@ -148,23 +148,51 @@ static int getfield (lua_State *L, const char *key, int d) {
   return res;
 }
 
+// Borrowed from Lua 5.2.4 (https://www.lua.org/source/5.2/loslib.c.html)
+#define LUA_STRFTIMEOPTIONS \
+        { "aAbBcCdDeFgGhHIjmMnprRStTuUVwWxXyYzZ%", "" \
+          "", "E", "cCxXyY",  \
+          "O", "deHImMSuUVwWy" }
+static const char* checkoption(lua_State* L, const char* conv, char* buff) {
+    static const char* const options[] = LUA_STRFTIMEOPTIONS;
+    unsigned int i;
+    for (i = 0; i < sizeof(options) / sizeof(options[0]); i += 2) {
+        if (*conv != '\0' && strchr(options[i], *conv) != NULL) {
+            buff[1] = *conv;
+            if (*options[i + 1] == '\0') {  /* one-char conversion specifier? */
+                buff[2] = '\0';  /* end buffer */
+                return conv + 1;
+            }
+            else if (*(conv + 1) != '\0' &&
+                strchr(options[i + 1], *(conv + 1)) != NULL) {
+                buff[2] = *(conv + 1);  /* valid two-char conversion specifier */
+                buff[3] = '\0';  /* end buffer */
+                return conv + 2;
+            }
+        }
+    }
+    luaL_argerror(L, 1,
+        lua_pushfstring(L, "invalid conversion specifier '%%%s'", conv));
+    return conv;  /* to avoid warnings */
+}
+
 
 static int os_date (lua_State *L) {
-  const char *s = luaL_optstring(L, 1, "%c");
-  time_t t = luaL_opt(L, (time_t)luaL_checknumber, 2, time(NULL));
-  struct tm *stm;
-  if (*s == '!') {  /* UTC? */
-    stm = gmtime(&t);
-    s++;  /* skip `!' */
-  }
-  else
+    const char* s = luaL_optstring(L, 1, "%c");
+    time_t t = luaL_opt(L, (time_t)luaL_checknumber, 2, time(NULL));
+    struct tm tmr, * stm;
+    if (*s == '!') {  /* UTC? */
+        stm = gmtime(&t);
+        s++;  /* skip `!' */
+    }
+    else
     
 #ifdef NXS_LIB
     stm = nx_localtime(&t);
 #else
     stm = localtime(&t);
 #endif
-
+   
   if (stm == NULL)  /* invalid date? */
     lua_pushnil(L);
   else if (strcmp(s, "*t") == 0) {
@@ -180,22 +208,22 @@ static int os_date (lua_State *L) {
     setboolfield(L, "isdst", stm->tm_isdst);
   }
   else {
-    char cc[3];
-    luaL_Buffer b;
-    cc[0] = '%'; cc[2] = '\0';
-    luaL_buffinit(L, &b);
-    for (; *s; s++) {
-      if (*s != '%' || *(s + 1) == '\0')  /* no conversion specifier? */
-        luaL_addchar(&b, *s);
-      else {
-        size_t reslen;
-        char buff[200];  /* should be big enough for any conversion result */
-        cc[1] = *(++s);
-        reslen = strftime(buff, sizeof(buff), cc, stm);
-        luaL_addlstring(&b, buff, reslen);
+      char cc[4];
+      luaL_Buffer b;
+      cc[0] = '%';
+      luaL_buffinit(L, &b);
+      while (*s) {
+          if (*s != '%' || *(s + 1) == '\0')
+              luaL_addchar(&b, *s++);
+          else {
+              size_t reslen;
+              char buff[200];  /* should be big enough for any conversion result */
+              s = checkoption(L, s + 1, cc);
+              reslen = strftime(buff, sizeof(buff), cc, stm);
+              luaL_addlstring(&b, buff, reslen);
+          }
       }
-    }
-    luaL_pushresult(&b);
+      luaL_pushresult(&b);
   }
   return 1;
 }