I'd keep the rule of never having magic numbers.

While

 seconds = num_days * 24 * 60 * 60

Is perfect readable most of the time, after coding 10 hours a day for tree or four weeks when in crunch mode

 seconds = num_days * HOURS_PER_DAY * MINUTES_PER_HOUR * SECONDS_PER_HOUR

is much easier to read. FrustratedWithFormsDesigner suggestion is better:

 seconds = num_days * DAYS_TO_SECOND_FACTOR

or even better

 seconds = CONVERT_DAYS_TO_SECONDS(num_days)

Things stop being obvious when you're very tired. *Code defensively*.