at〜, in〜

atとinは時間を表す前置詞

atはat 1:00のように時間の一地点を表す。
inはin an hourのように広い範囲を表す。

例文

Please finish cleaning the toilet at 2:00.
He always finishes breakfast at 8:00.
Please finish cleaning the toilet in an hour.
I finish lunch in an hour.

at, inを使った表現

He writes in his diary at the end of the day.
In the end, he said yes.結局、彼は承諾した
I can finish it in no time.それはすぐに終わらせることが出来る

時間ではないat

I would like to buy it at a discount.