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increase; grow, rise, get large, go up
decrease; decline, lessen, grow less, drop, fall, recede

A discrete "one" of something is the singular, you are talking about the "thing in itself".

Talking about anything other than "one" is talking not about the thing itself, but about an abstract number - a quantity - of that thing.

When talking about a quantity, you use the plural...I guess the logic is that "one of a thing" is just "a thing", not a quantity of the thing, and the "one of" bit is redundant "grammatical garnish", and not telling you anything.

"Two of a thing" is no longer equivalent to "a thing", it is "two things", which is a [quantity of] things.

"Half a thing" is not equivalent to "a thing" either, so again it is a [quantity of] things.

If it can't be simplified to "a thing", its plural.

The thing here is a second. 1 second = "the thing we call a second", but zero seconds, 0.5 seconds and 1.5 seconds don't = "the thing we call a second, so they are plural - they are "multiples" of a second, even if the multiplier is less than one.

Zero seconds, 0.5 seconds, 1 second, 1.5 seconds.


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