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.