diff --git a/LispLibrary.h b/LispLibrary.h
index b23d6f7..bff5e2a 100644
--- a/LispLibrary.h
+++ b/LispLibrary.h
@@ -172,19 +172,21 @@ Returns a list with all items for which tst is true removed from lst."
 Returns a list with all items for which tst is false removed from lst."
   (mapcan #'(lambda (x) (when (funcall tst x) (list x))) lst))
 
+(defvar *rtc-port* 1)
+
 (defun rtc-set (hr min)
  "(rtc-set hr min)
 Set the time on a DS3231 RTC. Times are in BCD, so use
 the appropriate reader macro, e.g. (rtc-set #x12 #x34)
 for 12:34. Assumes seconds are zero."
-  (with-i2c (str #x68)
+  (with-i2c (str *rtc-port* #x68)
     (write-byte 0 str)
     (write-byte 0 str)
     (write-byte min str)
     (write-byte hr str)))
 
 (defun rtc-get ()
-  (with-i2c (str #x68)
+  (with-i2c (str *rtc-port* #x68)
     (write-byte 0 str)
     (restart-i2c str 3)
     (reverse
diff --git a/ulisp-extensions.ino b/ulisp-extensions.ino
index 4645e38..c3734b9 100644
--- a/ulisp-extensions.ino
+++ b/ulisp-extensions.ino
@@ -2,6 +2,26 @@
  User Extensions
 */
 
+// Utility functions
+uint8_t
+dec_to_bcd(uint8_t n)
+{
+	uint8_t bcd = 0;
+	uint8_t	tens = n / 10;
+
+	bcd = tens << 4;
+	tens *= 10;
+	bcd += (n - tens) & 0x0f;
+	return bcd;
+}
+
+
+uint8_t
+bcd_to_dec(uint8_t n)
+{
+	return ((n>>4) * 10) + (n&0x0f);
+}
+
 // Definitions
 object *
 fn_now(object *args, object *env)