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(?).
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(?).
I feel that, this use case of でも is an abbreviated form of であっても。
(Note, I recognize there exist those uses of でも which do not fall into this category.)
So in here, もし時間がないなら、明日でもいいよ。 is an abbreviated (or colloquial) form of もし時間がないなら、明日であってもいいよ。 which translates to:
If no time, it is OK to be tomorrow.
Or something like that.
Please note that this is based on my recognition so when speaking in terms of Japanese strict grammar, it may not be precise.
One way to think of it, which I found in the Dictionary of Basic Japanese Grammar, is:
= 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.
I recently wrote a blog post on this actually! Rescheduling appointments without sounding like a jerk… 〜でも…?