Communicating with other servers is a good example. I need to have an accurate clock to within a few seconds when I sign Amazon S3 files. I have code that give

Communicating with other servers is a good example. I need to have an accurate clock to within a few seconds when I sign Amazon S3 files. I have code that gives access to restricted files on S3. To give access, the customer makes a request from my server, I authenticate, sign a new URL and redirect them to the signed S3 URL. For security reasons, the URL gets a signature that expires after a few seconds. The expiration date is based on X seconds from my servers clock. If my servers time differs from S3 server time then, according to S3, the link could be expired before it's even created. In theory, a redirect takes a second or less to complete so you'd think an expiration that's 5 seconds in the future would be enough. The reality is, over time, the server clock can differ as much as 1 minute or so from S3's clock. So, I just set the expiration to be 120 seconds into the future and call it a day.