One of the smaller, but very useful features in Windows Phone 8.1 Update 1 is Network Time Protocol (NTP) support. If you run a PC, NTP is nothing new, it is simply the ability for the computer to ping the server and pulldown the time off the internet. (Wikipedia fact: It is one of the oldest internet protocols in use!)
However, Windows Phone did not support this, at least not until Windows Phone 8.1 Update 1.
This lack of NTP support meant that devices without a network SIM users would manually have to set the date and time. In fact, devices that are powered off for an extended duration become discordant and lose their accuracy. Moreover, some cellular networks, mostly in Europe and a few other countries, do not support setting the date and time over a cell network, making this mandatory.
Now under the 8.1 Update 1, users get an option under Settings > Date + Time called 'Set time zone automatically.' All though that seems innocuous, combined with the new NTP support in the OS, the 'Set date and time automatically' feature now fetches the information over a Wi-Fi network if a SIM in not available. We tried it ourselves by removing the SIM, fudging the time and date, rebooting and then enabling the automatic feature. It instantly corrected the date and time, without the user having to do so.
Additionally, the 'set timezone' feature should help with Calendar entries when traveling, keeping your phone in sync.
Like we said, it is a small feature, but considering it is "one of the oldest internet protocols in use," long overdue in the Windows Phone OS.