How to print millis in [hh:mm:ss] format?

Projects Programming
6

Here is a clock sketch that CrossRoads wrote.
I modified it for 24-hour time.
Shouldn't be too hard for you to modify for what you have in mind.

/*  
    CrossRoads Timeclock  
    Adapted for 24hr time
    by Me!  08SEPT2012
    __h__m__s
    and leading zeros, too
*/

unsigned long previousTime = 0;
byte seconds ;
byte minutes ;
byte hours ;

void setup()
{
  //  Preset HHMMSS for 24hr rollover test
  //  Could jump to a function to set time
  //  and start
  seconds = 45;
  minutes =  59;
  hours =  23;
  Serial.begin (9600);
}

void loop ()
{
// I wonder whether "microseconds"...
  if (millis() >= (previousTime)) 
  {
     previousTime = previousTime + 1000;  // use 100000 for uS
     seconds = seconds +1;
     if (seconds == 60)
     {
        seconds = 0;
        minutes = minutes +1;
     }
     if (minutes == 60)
     {
        minutes = 0;
        hours = hours +1;
     }
     if (hours == 24)
     {
       hours = 0;
     }
     if (hours < 10)
     {
       Serial.print("0");
     }
     Serial.print (hours, DEC);
     Serial.print ("h");
     if (minutes < 10)
     {
       Serial.print("0");
     }
     Serial.print (minutes,DEC);
     Serial.print ("m");
     if (seconds < 10)
     {
       Serial.print("0");
     }
     Serial.print(seconds,DEC);
     Serial.println("s");
  } // end 1 second
} // end loop