Telling the time and dates

Put a time expression in the accusative and it means on, or for, with nothing else needed. Lundon is on Monday, la tutan tagon is all day, tri tagojn is for three days, la unuan de marto is on the first of March. English needs a preposition for each of those and picks a different one almost at random. Esperanto uses the ending it already had lying around.

They are prepositions, so they want a noun. To put a whole clause after them you need a helper word: antaŭ ol li venis for before he came, post kiam li venis for after he came. Dum and ĝis need nothing at all. Clock time meanwhile is the ordinal with la, so estas la tria and je la tria, and la tria kaj duono is half past three, counting forwards from the hour that struck rather than towards the next one.