I can't help but notice a similarity between sentences containing the construction you've isolated (indefinite + adjective + unit of measurement) and sentences containing collective or group nouns like 'committee'. They both can appear with indefinite articles in singular and plural contexts, for example:
It was an amazing two days. [singular]
They were an amazing two days. [plural]
It was an amazing committee. [singular]
They were an amazing committee. [plural]
Collective or group nouns are strange because sometimes they require verb agreement in the singular and sometimes they require it in the plural, for example:
- The committee is going to New York. [singular]
- The committee are eating with their families tonight. [plural]
It depends on whether the situation being described involves the committee acting as a unit or as individuals.
It seems like a similar duality exists with seemingly plural noun phrases of time and measurement (for example, 'two days'). They, like collective nouns, can be conceptualized as referring to a holistic unit or as a collection of discrete individuals. The conceptualization as a unit licenses the use of the indefinite.
To see that 'two days' exhibits a similar duality, just consider that this phrase, taken bare, can (seemingly) license both a singular and a plural verb:
- Two days is not enough. [singular]
- Two days are not enough. [plural]