By day, Robert Titterton is a lawyer. In his spare time though he goes
on stage beside pianist Maria Raspopova—not as a musician but as her page
turner. “I’m not a trained musician, but I’ve learnt to read music so I can
help Mariain in her performance.”
上面一段摘自2021年新高考英语全国卷I的阅读理解B。请问第一句开头的 by day 是什么用法?这个 by day 可以换成 in the day, during the day 吗?