Many security protocols use credentials that are only valid for a brief time. Here is a simple example of a problem caused by my server's time being more than 10 minutes off from the time on S3:
Yep, security is a big one. Authentication & encryption can behave very unpredictably if your times are not in sync. So, you might not be able to login to a server, or connect via HTTPS etc
http://illuminatedcomputing.com/posts/2015/04/paperclip_expi...