I'm not sure what you meant "days", did you mean a plural form of "day" or just a word "days" ? Well, the plural form of day is "날들", and the word "days" is "낮에(는)" day by day is "나날이", "날이갈수록"

하루 하루 means day by day, I know this from the korean singer ailee she has a song named day by day.