noun, adverb
one day; a single day
Dated noun/adverb for 'one day'. In modern Japanese, 一日 read いちにち is the usual word. ひとひ appears in classical or poetic contexts.
ひとひの旅路を終えて、宿に着いた。
After a one-day journey, we arrived at the inn.
adverb, noun
Dated adverb/noun for 'all day'. Modern Japanese uses 一日中 (いちにちじゅう) or 終日 (しゅうじつ). ひとひ in this sense is literary or old-fashioned.
ひとひ庭で遊んで過ごした。
We spent the whole day playing in the garden.
noun
Poetic noun for the first day of the month. In modern Japanese, 一日 read ついたち is the standard word. ひとひ is used in classical poetry or stylized writing.
弥生のひとひ、花が咲き始めた。
On the first day of the third month, the flowers began to bloom.