This is the 7754th most frequent Korean word.
하루에
"In a day" or "per day."
In this sentence, '하루에' is used to indicate the frequency in a time span, specifically 'in a day'.
나는 하루에 한 번 운동해요.
I exercise once a day.
Here, '하루에' specifies the time reference about 'in one day' for the question regarding hours worked.
하루에 몇 시간을 일하세요?
How many hours do you work in a day?
In this instance, '하루에' refers to the context of daily routine, meaning 'in a day' for maintaining a healthful schedule.