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I am reading Kanon, I will first give the context:

> 大体{だいたい｝、いつからこの家｛いえ｝の朝食｛ちょうしょく｝は和風｛わふう｝になったんだ…  
>  [Generally,] Since when did we start eating Japanese food for breakfast...

The main character then narrates:

> 昨日｛きのう｝までは間違いなく｛まちがいなく｝、トーストにゆで卵｛たまご｝だったはずだ  
>  Until yesterday, I have unmistakable expectation that it was toast and
> boiled eggs

If the expectation was until yesterday, should it not be 「だったはずだった」? He surely
no longer has the expectation since he has just had his expectation
contradicted. My belief is either I am confused at what the copulae are
referring to, or 「までは」 forces the rest of sentence into the past (so the plain
copula is present tense... in the past... (time travel tense trouble))
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2016-06-27T12:26:43.080
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copula
Confusing use of plain copula （だ should be だった?）
Does it make more sense to you if I reorganize your translated sentence like
this?

> I have unmistakable expectation that **it was toast and eggs until
> yesterday**.

The way I see it, getting something else today doesn't contradict his strong
belief that _before_ today it was always toast and eggs. He still has that
belief (not sure if "expectation" is an appropriate word here, I think that's
taking things too literally).
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2016-06-27T14:24:42.473
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## 筈{はず}だ is Expressing his _Current_ Expectation

If you remove はずだ this sentence only states a fact, as if the speaker is
explaining _**the situation**_ :

> 昨日までは間違いなく、トーストにゆで卵 **だった** 。
```

    Until yesterday it was always toast and boiled eggs.  
    
```

By adding 筈{はず}だ it sounds as if the speaking is explaining _**his
expectation**_ :

> 昨日までは間違いなく、トーストにゆで卵だった **はずだ** 。
>
> Until yesterday it was always _**supposed to be**_ toast and boiled eggs.
> (so what happened?)
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「昨日までは **間違いなく** トーストにゆで卵 **だったはず** だ」 is like 「（私の記憶では）昨日までは **絶対に** トーストにゆで卵
**だった** 」, "I'm sure it was toast and boiled eggs / It must have been toast
and boiled eggs until yesterday (as far as I can remember)".

Compare:

> 「XX **だったはず** だ」 -- "it **must have been** XX" "I **am** sure it **was** XX"  
>  vs  
>  「XXの **はずだった** 」 -- "it should have been XX" "it was supposed to be XX"

* * *

By the way, the だいたい means そもそも, "to begin with" or "in the first place", not
"generally". See [definition #2⃣-2 of 「だいたい」 in
デジタル大辞泉](http://dictionary.goo.ne.jp/jn/133823/meaning/m0u/):
「もとはと言えば。そもそも。『だいたい言い出したのは君だよ』」

(And, as you might have already noticed, the いつから～～なったんだ here is a rhetorical
question (修辞疑問文) showing the speaker's dissatisfaction or irritation, not a
genuine question expecting an answer.)
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