zeropad(number, digits)
Number = the number you want to pad. In your case, this would be the variable representing the hour/minute/second/etc.
Digits = the total number of digits you want displayed.
For example:
zeropad(5,2) results in "05".
zeropad(141,3) results in "141"
zeropad(101,10) results in "0000000101"