We're all aware that magic numbers (hard-coded values) can wreak havoc in your program, especially when it's time to modify a section of code that has no comments, but where do you draw the line?

For instance, if you have a function that calculates the number of seconds between two days, do you replace

seconds = num_days * 24 * 60 * 60

with

seconds = num_days * HOURS_PER_DAY * MINUTES_PER_HOUR * SECONDS_PER_MINUTE

At what point do you decide that it is completely obvious what the hard-coded value means and leave it alone?