Im thinking about a context where i could use "it is afternoon" in portuguese. But, "it is late" is the first translation that came up to my mind...
I immediately imagined it as the first sentence spoken by the narrator at the beginning of an expository documentary segment. "It is afternoon. The lionesses rest in the shade of . . ." etc. ^_^
So does "É dia" mean "It's early" and "É noite" mean "It's really late"? ;)
I wouldn't put it like that since "dia" and "noite" work most of the times as nouns or adverbial adjuncts of time:
É dia = it's day / É de dia [de manhã] = It's (in the) morning
É tarde = it's late / É de/à tarde = It's (in the) afternoon
É noite = it's night / É de/à noite = It's (in the) evening
É (muito) cedo = it's (really) early
É (muito) tarde = it's (really) late
É melhor ir embora para casa, é tarde (you'd better go home, it's late) / é noite e não quero voltar sozinho (it's late and I do not want to go back alone).
I agree with you in the first sentence, but the second one would be better expressed as "It's late night and I don't want..." = "É tarde e não quero..." or "É tarde da noite e não quero...".