What about singular and plural anything?

What about singular and plural anything?

Indefinite pronouns, something, anything are singular

1.Winter is the time of year the days _____ are short and nights are long.(when or that) why?
2.Hi you look tired.
I am tired.I______ the living room,all day.
A.have been painting B.have painted
The teacher said choose a, can't judge?

The first one is because it's an attributive clause and it lacks a subject. The second one is because it's something I've been doing all day and there's no time in the past for comparison

Three and a half days () not a short time A is B are

A
Time, distance, weight, money and so on are regarded as a whole when they are used as subjects