Why is a measure word not required in 一年有三百六十五天?

Question

Consider the following two sentences:

A year has twelve months
一年有十二個月
_{yī nián yǒu shí’èr gè yuè}

A year has three hundred and sixty-five days
一年有三百六十五天
_{yī nián yǒu sānbǎi liùshíwǔ tiān}

Why is the measureword 個 left out in the second sentence? What is the general grammar rule for cases like this?

Answer 1

Some nouns need a classifier, some don't; 月(month) needs one but 天(day) doesn't.

一年有十二個月 - 月(month) needs a classifier

一年有十二個月份 - 月份(month) needs a classifier

一年有五十二週 - 週(week) doesn't need a classifier

一年有五十二個星期 - 星期(week) needs a classifier

一年有三百六十五天 - 天(day) doesn't need a classifier

一年有四季 - 季(season) doesn't need a classifier

一年有四個季節 - 季節 (season) needs a classifier

一小時有六十分鐘 - 分鐘(minute) doesn't need a classifier

一分鐘有六十秒 - 秒(second) doesn't need a classifier

Generally, if a noun can be a classifier itself, it usually doesn't need a classifier. 週, 天, 季, 分鐘 and 秒 can all be classifier

Example: 三週工資, 兩天比賽, 一季收成, 幾分鐘路程, 幾秒時間

Answer 2

A year has twelve months, 一年有十二個月。

A year has three hundred and sixty-five days, 一年有三百六十五天。

The question is when to use the measure word 個, but when to not use it?

It depends on a couple of factors.

We say 一年有十二個月 but not say 一年有十二月. It is because 十二月 is the twelfth month of the year (December). That is why you have to say 十二個月 to avoid confusion.

You can say 二月有二十八天。No need to say 二月有二十八個天。The 28th day of the month is 二十八号。

Another example of using 個, 这个月有二十一個工作日。

Also, 邓丽君演唱的《你怎么说》，“三百六十五個日子不好过，你心里根本没有我”.

Answer 3

三月 -> March

三个月 -> Three months

The usage without "个" is occupied by special meanings (a time point) so the "个" is added to express a period of time.