What is the purpose of 制 in this sentence 夏时制开始
From speaking to many mainland Chinese, 夏时 means daylight savings. If a Chinese speaker knows about daylight savings, 夏时 is quite clear. 夏时制 is more clear especially for Chinese who aren't familiar with daylight savings. The three words together express "system of daylight saving" loose translation. 

Honestly, would a non-American English speaker be able to know what daylight savings is without previous knowledge of the concept? I don't think so. I can imagine less worldly Chinese not understanding either of the two above phrases unless the concept was carefully explained to them.