It says “one day” so I wondered if it could also be used in a sentence like “I’ll become a hokage one day” or is there something else for that?
一日 - the time interval of 1 calendar day.
一日 - the first day of the month.
いつか - someday, one day (in the future). There’s a kanji version for this but it’s almost never used: 何時か. Not to be confused with 五日 (five days, fifth day of the month).
Thanks for the help!
一日 can also be used adverbially.