"Does anyone really know what time it is? Does anyone really care?" It's a fine song lyric for Chicago, but when it comes to computers, the answer is, "Yes, we really care. We, really, really care that our all are computers' servers logs, e-mail date and time stamps, and our backups agree on the date and time."
How do you do that, though? The usual answer on SOHO (Small Office/Home Office) networks is to have each PC call in over the Internet to a NTP (Network Time Protocol) server. That's fine, but it does waste a few bits of network bandwidth and with any modern Linux you can set up your own personal NTP server for your PCs.