'30 days or less' or '30 days or fewer'

[WixingMords]

Countable or plurals are general paired with fewer aren't they?

But isn't there an exception with measurements?

Help!!

[HippyChickMama]

I would say '30 days or less' or 'fewer than 30 days'.

[MrsHathaway]

It's not whether you can count them, but whether you can only count them.

You need context for days.

For example, days are discrete for visas - if you are intending to spend fewer than ninety days in Hathawayland you do not require a visa.

But they are continuous in astronomy - a Mars year is less than 700 days.