adverb
at any time; at any moment
Emphasizes that something could happen at any moment, often with a sense of urgency or unpredictability. Used in warnings or when describing a situation that may change suddenly.
地震はいつ何時起こるかわからない。
An earthquake could happen at any time.
彼はいつ何時倒れてもおかしくない状態
He is in a condition where he could collapse at any moment.
adverb
when; at what moment
More emphatic than いつ alone; often used in rhetorical questions or to express impatience, demanding a specific time. Can sound pressing or dramatic.
いつ何時になったら準備ができるんだ？
When on earth will you be ready?
いつ何時まで待てばいいのか教えてくれ。
Tell me until when exactly I should wait.
いつ is the neutral, everyday word for 'when'. いつ何時 is more emphatic and carries a nuance of urgency or impatience, often used in warnings or pressing questions.
いつでも means 'anytime' in a permissive or unconditional sense (e.g., 'come anytime'). いつ何時 in sense 1 is more about an unpredictable, possibly negative event that could occur at any moment.
A compound of いつ (when) and 何時 (なんどき, what time), with the second element reinforcing the interrogative meaning. The exact historical derivation is uncertain, but the form likely developed as an emphatic expression.