'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!!

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

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.