day by day; with each passing day
Emphasizes gradual change or progression over days. Often used in written or slightly formal contexts.
日一日と春が近づいているのを感じる。
I feel spring approaching day by day.
彼の病状は日一日と悪化していった。
His condition worsened with each passing day.
A fixed adverbial phrase composed of 日 (day) repeated with the particle と, literally 'day by day'. The reading ひいちにちと is a regular combination of ひ (day) and いちにち (one day).