"My date on Sunday was a little boring."
つまら なかった でした sounds like "It was was boring". We also never say ない でした, but we say ない です in present tense.
PRESENT: 1. つまり ません。 2. つまら ない です。 3. つまら ない。(*)
PAST: 1. つまり ません でした。 2. つまら なかった です。 3. つまら なかった。(*)
- is usually in written sentences or in colloquial.
つまらない WORD ORIGIN: 『つまる + ない → つまらない』 Denying form of old Japanese つまる(verb), which is that it never get to the end, conclusion. It never get any answers or anything, so that is to be BORING. つまり (therefore) is originated from the same verb, つまる.
In Japanese it's always the adjective itself that is conjugated (in negation or past tense), never the additional です。
You can think of つまらなかった です (litt. "it was boring, it is") as saying "It is (so/such/a fact that) it was boring": a present tense statement about something that happened in the past. On the other hand, saying つまらない でした (litt. "it is boring, it was") is like a past tense statement about something that currently happens... it's odd.
To emphasize something Alcedo-Atthis mentioned (and I read somewhere), if you can conjugate something that isn't the "です", then you should. In this case, you can conjugate the adjective as つまれなかった, and therefore you should. Changing the です to でした is then redundant and therefore incorrect.