Translation:A minute is less than eighty seconds.
I prefer the Oxford description: Use fewer if you’re referring to people or things in the plural (e.g. houses, newspapers, dogs, students, children). Use less when you’re referring to something that cannot be counted or does not have a plural (e.g. money, air, time, music, rain).
However, that being said, both are grammatically correct and interchangeable. It was only in the 1700s that the grammarian Baker made a comment regarding the word "less". In the comment he uses the phrases "I should think" and "appears to me" to indicate the guideline his personal preference as opposed to a strict grammar rule.
In other words, when you hear a millennial speaking and not following Baker's suggestion, just remind yourself that when you speak, more people will listen, not because the millennial is wrong and you are right but simply because it sounds better. :)
I'd be comfortable with "a minute has fewer than 80 seconds" or "a minute is less than 80 seconds." Incidentally, it seems to me that the non-pedantic often use "less" instead of "fewer" but seldom use "fewer" instead of "less," as though having fewer words would make a language simpler and more international.
Probably because you used the word "long". Remember, according to Einstein's theory, time is relative and is not an object that can be measured with a ruler.
You could probably say "a minute has less than..." and it would be marked as correct. However, if you tried something like "a minute is comprised of less than ..." then I think it would be marked as wrong. Although they both mean the same thing just remember, you're being marked by a computer. :)