The character 一日 can be read as “いちにち” (“the whole day”), or as ついたち (“the first day of the month”).
Depending on the context, the readings of some Kanji differ.

Furigana are a Japanese reading aid. They are Hiragana characters that are written next to or above a Kanji in Japanese writing to indicate its pronunciation.

Conventional Furigana software does not recognize the holistic meaning or context of a text, and is therefore sometimes unable to provide the Furigana in the way a Japanese reader would actually read the text.

However, with AI, this is now possible, which is why I programmed this software.
“Smart-Furi” analyzes the text for context, tone, etc., to add the appropriate readings as Furigana to the text – so that one does not learn the readings of the characters incorrectly, but as if a Japanese person were reading the text aloud.