Why are numbers exempt from the less vs. fewer rule?

The way I tend to apply the less vs. fewer rule is:

• If I can count it, it's fewer - (Drink fewer glasses of water.)
• If I can't count it, it's less - (Drink less water.)

But when it comes to numbers and time, I usually see less where either less or fewer seem to apply, for example:

• Less than 24 hours ago
• Fewer than 24 hours ago
• 10 items or less
• 10 items or fewer

I can count hours and items, and I can count to 24 and 10, but I can't count a 24 or 10.

Even though I see less applied, which is actually correct?

Solution 1:

When a phrase beginning with a number denotes an uninterrupted sequence, eg. ten minutes, ten miles, ten kilos, it is the sequence as a whole that is being modified. Hence, "less than ten minutes" means "in a time shorter than a ten-minute duration."

Where the minutes are not contiguous, however, and we are actually counting them, then "fewer" can be used, as in "exercise breaks last up to a minute; in one hour, take fewer than ten minutes." If the time is one uninterrupted unit, we would say "take less than ten minutes."