DM Entry: 1 Day vs 24 hours

Why do some effects describe their duration in multiples of 24-hour units and others in multiples of 1-day units?

This seems to say the same thing. But actually there is a distinction.

Any item, spell, or effect described with a duration in days (such as "charges per day") counts the day beginning at dawn each day. Rules Compendium page 86

If you have an item with 3 charges per day, it renews its charges each day at dawn.

A spell that lasts 3 days will end at the 3rd dawn after casting.

Items, spells, and effects that have a duration listed as "24 hours" or some multiple thereof last only that many hours before the effect expires, counted from the time of activation, regardless of the actual time of day.

So a spell that lasts "24 hours per level" cast by a 3rd level caster that's cast at 4pm will end at 4pm 72 hours (or 3 days) later.