Rashi employs the methods of idioms. An idiom is a group of words whose meaning transcends the meanings of the individual units. Most people think of the dictionary as listing meanings of individual words. However a dictionary should also list the meanings of idioms, groups of words.

Verse Ex21-20:21 should be translated as follows And if a man smite his bondman, or his bondwoman, with a rod, and he die under his hand, he shall surely be punished. Notwithstanding if he survives for 24 hours he shall not be punished; for he is his money.

Advanced Rashi: The above verses states that there is a death penalty for killing one's slave provided the slave dies within 24 hours. The actual Biblical verse does not use the term 24 hours. Rather it uses the literal term day or days. Rashi identifies this as an idiom: The Biblical day or days is an idiom meaning 24 hours. The etymology of the idiom is a uniform period that corresponds to a day but can span two days.