Wirelessly get the current date and time
There are a number of signals being broadcast that contain date and time information.

As @andrejaKo mentioned there is WWV which is a radio station run by the National Institute of Standards and Technology. This station broadcasts on several frequencies that can be picked up with a general coverage shortwave receiver. 5, 10, 15, and 20 MHz come to mind. There are modules available which can receive this signal and convert it into digital data for MCU interfacing.

Second is the GPS satelite system. There are receivers that can pick up GPS signals and provide time base information for computers.

Third, some broadcast radio and TV signals have digital information embedded, which contains date and time information.

Mobile phones always seem to know what time it is. I don't know if you can get that information from the signals transmitted by the mobile phone network, or not, unless you have a device with a mobile phone service plan.