Paul speaks of his conscience as if it could be physically clean. This means that a person does not feel guilty because he has always tried to do what was right. Alternate translation: "knowing I have tried my hardest to do what is right" (See: [[rc://en/ta/man/translate/figs-metaphor]])
Here "night and day" are used together to mean "always." Alternate translation: "Always" or "Constantly" (See: [[rc://en/ta/man/translate/figs-merism]])