In a microcontroller powered by a solar cell we need to have the current time. When the sun decrease the micro-controller switch off and all the volatile data vanish. When the sun increase the micro-controller reboot and we need to set the time on reboot.

I had an idea but I do not know enough wifi to be sure it's feasible : Getting the current time from the surrounding wifi signals. 

In this post [How much information can be gathered about a network without connecting?][1] you can get :  

 - ESSIDS BSSIDS 
 - Crypto strength and authentication scheme (Pre shared
   Key vs EAP/Radius) 
 - Signal strength
 - ....

But I can't see if the AP current time is obtainable ? 

  [1]: https://security.stackexchange.com/questions/102587/how-much-information-can-be-gathered-about-a-network-without-connecting