Shell Programming and Scripting

Ok, this is going to be hard to describe, but here it goes.

I have written a bash script that, while executing starts a timer, and when done stops the timer. The $RUNTIME variable value is in seconds, so the variable usually equals a number like 126 (equals 2 minutes 6 seconds). In my script I want to format the variable into something like the output of "date "+%H:%M:%S" (usually a time formatted as HH:MM:SS). I have writtin/borrowed a function that almost does what I want, except that when called, it does not prepend a "0" to any value that is only a single digit, so my function returns values like:
12:56:17 (all two digit values seperated by colons)
or
1:5:18 (single digits do not have the preceding "0")
I want to force the output of this function to always display all values as two character numbers (ie 01, 02, 07...). Here is my function:
secondsConvert ()
{
RUNTIME=`expr $STOP - $START`
TEMP0=`expr $RUNTIME / 60`
SECOND=`expr $RUNTIME - $TEMP0 \* 60`
TEMP1=`expr $TEMP0 / 60`
MINUTE=`expr $TEMP0 - $TEMP1 \* 60`
TEMP2=`expr $TEMP1 / 24`
HOUR=`expr $TEMP1 - $TEMP2 \* 24`
echo ""$HOUR":"$MINUTE":"$SECOND""
}
So again, how to I force the value of the variables $HOUR, $MINUTE, &$SECOND to always output two digits, even if the hour is less than 10? I thought I could pass the values to sed, then allow sed to convert single digit numbers into two digit numbers, but to be honest, I don't have any clue how to do that.

So, just to clarify all the above, I'm looking for a function that will take a value in seconds and convert it to the same format as the date command (HH:MM:SS).

I apologize if this makes no sense, as I am new at this, and this was hard to explain. Please any feedback is appreciated.

without going on the tangent of re-writting the function....
one way......

Code:

printf "%02d:%02d:%02d" "$HOUR" "$MINUTE" "$SECOND"

Thanks, that worked perfectly. BTW I did rewrite my function as follows, including your input:
elapsedTotal ()
{
RUNTIME=`expr $STOP - $START`
HOURS=`expr $RUNTIME / 3600`
REMAINDER=`expr $RUNTIME % 3600`
MINS=`expr $REMAINDER / 60`
SECS=`expr $REMAINDER % 60`
printf "%02d:%02d:%02d\n" "$HOURS" "$MINS" "$SECS"
}
Thanks again!

You can use the shell for integer arithmetic instead of expr, e.g....

Code:

printf "%02d:%02d:%02d\n" $((RUNTIME/3600)) $((RUNTIME/60%60)) $((RUNTIME%60))