a whole month; an entire month
Emphasizes the full duration of one calendar month or roughly thirty days, often used when the length feels significant.
It hasn't rained for a whole month.
It took a full month to finish this work.
Compound of 丸 (まる, 'whole, complete') + 一月 (ひとつき, 'one month'). The reading ひとつき for 一月 is a native Japanese counting form.