標點3-am和pm要有period嗎?

我認為要有，像以下所說：
http://www.businesswritingblog.com/business_writing/2009/06/what-is-the-correct-time-am-pm-am-pm-am-pm-.html

有些人認為不用：
http://www.eslcafe.com/grammar/the_period04.html

但無論如何，am和pm與之前的時間要留有space:
http://www.quickanddirtytips.com/education/grammar/daylight-saving-time-0

雖然也有style guide接受沒有space:
http://english.stackexchange.com/questions/22784/am-pm-vs-a-m-p-m-vs-am-pm