half an hour，是一个整体概念，自是单数
例句：
It'll be ready in half an hour's time.
另，"one and a half +名词"作主语时，谓语动词要用单数.例如:One and a half apples is left on the table.桌子上有一个半苹果.
half an hour在英语里的意思是半个小时
半个小时是30 miuntes ，所以是单数
复数名词表示时间，价格等时，表示单一的概念，谓语动词要用单数形式.
例句:
Three months is a long time.
Ten minutes seems an hour whe one is waiting for a phone call
