Milliseconds using square wave RTC

i am trying to make a code that prints the time hh:mm:ss:ms using RTC,i have made it using millis() but it is not accurate +/-2 milliseconds

ho can i make it using sq wave and intterupt

here is the code that i have used

#include <Wire.h> #include "RTClib.h" RTC_DS1307 RTC; // This is the DS1307 hardware RTC RTC_Millis SoftRTC; // This is the millis()-based software RTC long startMS; // This will hold the start time in milliseconds void setup () { Wire.begin(); RTC.begin(); // Connect to the DS1307 SoftRTC.begin(RTC.now()); // Initialize SoftRTC to the current time startMS = millis(); // get the current millisecond count } void loop() { DateTime now = SoftRTC.now(); long nowMS = millis(); Serial.print(now.year(), DEC); Serial.print('/'); Serial.print(now.month(), DEC); Serial.print('/'); Serial.print(now.day(), DEC); Serial.print(' '); Serial.print(now.hour(), DEC); Serial.print(':'); Serial.print(now.minute(), DEC); Serial.print(':'); Serial.print(now.second(), DEC); Serial.print(':'); Serial.print((nowMS - startMS)%1000, DEC); // print milliseconds Serial.println(); 

i tried to put this code that is based on ISR but with no luck

volatile time_t isrUTC; // ISR's copy of current time in UTC volatile time_t millisecondsFromISR; // ISR's copy of current time millisecond offset void incrementTime() { millisecondsFromISR = millis(); //do first to get most accuracy ++isrUTC; } int getMilliseconds() { // need to make an atomic copy: nointerrupts(); unsigned long t = millisecondsFromISR; interrupts(); return millis() - t; }