Could the time be retrieved from the wi-fi router on wich device is connected ? That would be awesome !

A lot of projects may not connect through a router, they may be simple WiFi points directly to the WiFi of the mobile device, such as ESP8266 boards.

You can connect to the device from different phones with different time.

Are you saying that you can do this simultaneously ?  How can this work ?  Surely the data coming into your sketch will be in conflict all the time ?  I can't envisage a scenario in which you would want to do that.....

I think it most likely the phones would have the same time, so the time from any connected phone or tablet would be sufficient for most people's needs.

The phone has this info, the controller may need it, so it would be great if this info could be included in the GUI interface from the phone/tablet to the controller.  Yes, I understand that the collection of this data may be different for Android and iOS, but I'm sure there would be a way around this.

Having this info available would mean that the control system would not need a battery-backed RTC, as the phone would continuously update when in a reception area, and I'm sure they continue to maintain time reasonably accurate time when they are not.

In my case accuracy to 1 second would be enough, but even if it were "last known" date and time it would be sufficient.