Chinese Language Stack Exchange is a question and answer site for students, teachers, and linguists wanting to discuss the finer points of the Chinese language. It's 100% free, no registration required.
It literally means differ. 差一刻九点 means a quarter to nine. 差 in the phrase 差一点 and 差几块钱/几个月 means the same, that is, adding the amount missed will turn the current situation into the target situation (could be better or worse).