I, too, was confused by the catch 22 of wanting to put the date of A/W certification on the data plate but also needing to have the plate engraved and installed ahead of the inspection.
So...I scrutinized the documentation from Van's and on the FAA Form 8130-15 (block 4) is a "Date of Manufacture" attested to by Van's along with serial number, etc.
That is the date I put on my data plate, but like Mel says, you can put what you want. I just wanted a "Date of Manufacture" that appeared on an official form somewhere that I could point my finger to.
If the FAA "official" date of manufacture is the date of A/W compliance then they would probably know why it is different than what is requested on the official Form 8130-15 as certified by the manufacturer. I don't.
Tony