At 1st it appears ' simple ' as you say , but the Grande Date complication is a considerable piece of work in its own right .
There was an excellent article on the Grande Date written a while back by Geoff Kingston in Blancpain's Lettres from Le Brassus magazine .
I'm sure it would come up in a Google search or could be downloaded in softcopy from Blancpain's website .
The movement is also a visual delight through the see through case back , and the size is just right ,
I'm not a big fan of wristwatches that are so big they could double as wall clocks 
Regards .