diff --git a/DS3231/ds3231_pb.py b/DS3231/ds3231_pb.py
index f03d3cc..c98e43e 100644
--- a/DS3231/ds3231_pb.py
+++ b/DS3231/ds3231_pb.py
@@ -1,6 +1,7 @@
 # Pyboard driver for DS3231 precison real time clock.
 # Adapted from WiPy driver at https://github.com/scudderfish/uDS3231
 # Includes routine to calibrate the Pyboard's RTC from the DS3231
+# delta method now operates to 1mS precision
 # Adapted by Peter Hinch, Jan 2016
 
 import utime, pyb
@@ -24,9 +25,8 @@ def bcd2dec(bcd):
     return (((bcd & 0xf0) >> 4) * 10 + (bcd & 0x0f))
 
 def dec2bcd(dec):
-    t = dec // 10
-    o = dec - t * 10
-    return (t << 4) + o
+    tens, units = divmod(dec, 10)
+    return (tens << 4) + units
 
 class DS3231:
     def __init__(self, side = 'X'):
@@ -43,7 +43,7 @@ class DS3231:
 
     def get_time(self, set_rtc = False):
         if set_rtc:
-            data = self.await_transition() # For accuracy set RTC immediately after a seconds transition
+            data = self.await_transition()      # For accuracy set RTC immediately after a seconds transition
         else:
             data = self.ds3231.mem_read(7, DS3231_I2C_ADDR, 0) # don't wait
         ss = bcd2dec(data[0])
@@ -80,38 +80,46 @@ class DS3231:
             self.ds3231.mem_write(dec2bcd(MM), DS3231_I2C_ADDR, 5)
             self.ds3231.mem_write(dec2bcd(YY-1900), DS3231_I2C_ADDR, 6)
 
-    def delta(self):
-        return utime.time() - utime.mktime(self.get_time())         # No. of secs RTC leads DS3231
+    def delta(self):                            # Return no. of mS RTC leads DS3231
+        self.await_transition()
+        rtc_time = now()
+        rtc_ms = rtc_time[0] * 1000 + rtc_time[1]
+        t_ds3231 = utime.mktime(self.get_time())  # To second precision, still in same sec as transition
+        return rtc_ms - 1000 * t_ds3231
 
-    def await_transition(self):
+    def await_transition(self):                 # Wait until DS3231 seconds value changes
         data = self.ds3231.mem_read(7, DS3231_I2C_ADDR, 0)
         ss = data[0]
         while ss == data[0]:
-            data = self.ds3231.mem_read(7, DS3231_I2C_ADDR, 0)      # Await a seconds transition
+            data = self.ds3231.mem_read(7, DS3231_I2C_ADDR, 0)
         return data
 
 # Get calibration factor for Pyboard RTC. Note that the DS3231 doesn't have millisecond resolution so we
 # wait for a seconds transition to emulate it.
 # This function returns the required calibration factor for the RTC (approximately the no. of ppm the
 # RTC lags the DS3231).
-# For accurate results the delay should be at least 120 seconds. Longer is better.
+# For accurate results the delay should be at least two minutes. Longer is better.
 # Note calibration factor is not saved on power down unless an RTC backup battery is used. An option is
 # to store the calibration factor on disk and issue rtc.calibration(factor) on boot.
 
-    def getcal(self, seconds):
+    def getcal(self, minutes=5):
         rtc.calibration(0)                      # Clear existing cal
         self.save_time()                        # Set DS3231 from RTC
         self.await_transition()
         rtcstart = now()
         dsstart = utime.mktime(self.get_time())
-        pyb.delay(seconds * 1000)
+        pyb.delay(minutes * 60000)
         self.await_transition()
         rtcend = now()
         dsend = utime.mktime(self.get_time())
         dsdelta = (dsend - dsstart) *1000
         rtcdelta = rtcend[0] * 1000 + rtcend[1] - rtcstart[0] * 1000 - rtcstart[1]
         ppm = (1000000* (rtcdelta - dsdelta))/dsdelta
-        return -ppm/0.954
+        return int(-ppm/0.954)
 
-    def calibrate(self, seconds):
-        rtc.calibration(self.getcal(seconds))
+    def calibrate(self, minutes=5):
+        print('Waiting {} minutes to acquire calibration factor...'.format(minutes))
+        cal = self.getcal(minutes)
+        rtc.calibration(cal)
+        print('Pyboard RTC is calibrated. Factor is {}.'.format(cal))
+        return cal