兩句簡單的英文句子,請告訴我錯在哪裡?

1.Let me assure you that we'll get everything ready at what time you arrive.

2.Let me assure you that we'll get everything ready at which time you arrive.

請問這兩句分別錯在哪裡,解釋的越詳細越好

Update: 請告訴我為何不能用what 或 which
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