It may surprise you to learn that, in practice, clients’ clocks are heavily skewed. A recent study of Chrome users showed that a significant fraction of reported TLS-certificate errors are caused by client-clock skew. During the period in which error reports were collected, 6.7% of client-reported times were behind by more than 24 hours. (0.05% were ahead by more than 24 hours.) This skew was a causal factor for at least 33.5% of the sampled reports from Windows users, 8.71% from Mac OS, 8.46% from Android, and 1.72% from Chrome OS.