Rashi on Exodus 21:21/רש"י על שמות כ״א:כ״א

1אך אם יום או יומים יעמד לא יקם NOTWITHSTANDING IF HE CONTINUE A DAY OR TWO, HE SHALL NOT BE AVENGED — If the master is free of the death penally in case of the servant remaining alive one day, is it not logical that he is free if he lives two days (what need is there for Scripture to add יומים)? But it adds it to show that the term יום is to be defined by יומים — “a day that is like two days”. And what kind of day is thereby meant? A period of twenty-four hours (a day which comprises a part of two separate days) (cf. Mekhilta).

2לא יקם כי כספו הוא HE SHALL NOT BE AVENGED: FOR HE IS HIS MONEY — However, any other person who smote him (the servant) is subject to the death penalty although he lived 24 hours before dying.