I hereby modify my design and drop the bluetooth module. The communication need not be two-way. I suggest ultrasonics, assuming the phone's speaker is able. Now

I hereby modify my design and drop the bluetooth module. The communication need not be two-way. I suggest ultrasonics, assuming the phone's speaker is able. Now all energy is consumed in the phone. There will be only the E-paper-module and microphone and some filtering in the watch. Also the refreshing rate can now be faster. If the clock shows only the current date, the battery will last decades. The blanking timer is initiated by the presence of the said sonic carrier, or rather by the lack of it.