Longines uses an LED flashlight on the mobile to set the exact time on the watch. (see"
https://www.longines.com/universe/news/longines-presents-the-new-conquest-v-h-p-gmt-flash-setting) Its free application is intended for this purpose. When pressed the button on handy, the LED sends a light message to the watch. But this could also be used to set the time in Arduino applications. I attach a picture of what it looks like.
It's a simple sequence. Since it is also known what is in the message (time) it will be easy to decode it.
Can anyone do that?