grammar - Why でも is used rather than も in this sentence?

もし時間がないなら、明日でもいいよ。Why is the particle で used? With time expressions the で particle expresses time limit. So it's weird to see the で with 明日. 明日もいいよ seems better(?). でも here can be the "even though/even/but" meaning. But も sounds better(?).

Answer

One way to think of it, which I found in the Dictionary of Basic Japanese Grammar, is:

でも ＝ て-form of です/だ ＋ も

＝　verb meaning “to be”　＋　particle meaning “even”*

〜でもいい is the です/だ version of the grammatical pattern 〜てもいい, which we use when asking for permission to do things. By putting the verb です/だ in its て-form, we can then ask if the thing itself (a noun) is permissible.

• お手洗い行ってもいいですか？/ May I go to the bathroom? [literally: Is even going to the bathroom OK?]


• お手洗いでもいいですか？/ Is the bathroom OK? [literally: Is even it being the bathroom OK?] You’d say this if you’re doing something and asking if it’s OK to do it in the bathroom.

I recently wrote a blog post on this actually! Rescheduling appointments without sounding like a jerk… 〜でも…?