How does the Chinese Consulate calculate the three months your visa is valid?

Does the consulate count a month as 28 days and then they just give you 84 days of validity on your visa, or do they simply look at a calendar, round up the days on the month you applied and give you whatever is left? I hope this question makes sense, I know it sounds strange. But really, I just need to know exactly how much time is allotted for tourist visas.