'When he will arrive' is not a time clause.
"When will he arrive?" - We don't know when he will arrive. When he will arrive is anybody's guess.
"Where will he live?" - We don't know where he will live. Where he will live is anybody's guess.
"What will he do?" - We don't know what he will do. What he will do is anybody's guess.
Note that the expression is anybody's guess, not nobody's guess.
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