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 threehundreds sixtyfive 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?