noun
half a month
Refers to a period of about 15 days. Often used in scheduling or duration contexts.
この薬は半か月分あります。
This medicine is a half-month supply.
半か月後にまた来てください。
Please come back in half a month.
半月 (はんげつ) usually means 'half-moon' (shape), while 半か月 (はんかげつ) specifically means 'half a month' (duration).
一か月 means 'one month', while 半か月 means 'half a month'.
Compound of 半 (half) and か月 (counter for months). The か is a counter suffix derived from 箇.