1.
anytime is fine, whenever works
A casual expression indicating that the speaker has no preference about timing. Used when asked to choose a time for meeting, scheduling, etc.
来週ならいつでもいいです。
Any day next week works for me.
いつでもいい is composed of いつ (when) + でも (even/any) + いい (good/fine). It follows the same productive pattern as どこでもいい, なんでもいい, etc.
Common patterns
いつでもいい is often scoped by a preceding time frame:
Keigo
いつでもいいです or いつでも構いません are polite equivalents suitable for business or formal contexts.
Pattern: 〜でもいい
This is a productive pattern combining a question word with でもいい: