Time and again a new message appears in between the old ones. Of course I only realize that a lot later and by accident. Most of these stray messages are spam, but sometimes an important message comes with a wrong date, too.
This is why you should
- set your system's clock to the accurate time via the Net periodically.
Mac users have that ability built into their OS X and merely need to turn it on.
The same is true for users of Windows XP or later. Users of earlier Windows versions can employ free tools like Automachron or the NTPTime Client.
Linux/UNIX users can use NTP.
Time synchronization is available for almost any other platform, too.

