白天(bái tiān) - daytime, during the day in Mandarin

白天 (bái tiān) is the daylight half of the day, the stretch that sits opposite 晚上 and 夜里. It answers when, and it names the hours when the sun is up rather than a fixed clock time.

Like other time words it goes straight in front of the verb with no preposition at all: 我白天上班 needs nothing added. It can also sit at the head of the sentence for emphasis, as in 白天这条街很热闹, and it takes 的 when it modifies a noun: 白天的温度.

The trap is translating in the daytime word for word. There is no 在 in 我白天工作, and adding one marks the sentence out as learner Chinese instantly. Also keep 白天 apart from 天: 天 counts days as units, so three days is 三天, never 三个白天.