I concur with Claire, in that the number is an sequential order number and you'd need a lot of them, with accurate datings for them to be useful to date your garment.
The short answer is that somewhere in the YSL office and factory/s there would have been a book with the order details, but without that inside knowledge, it's difficult to guess at these things.
There are many other tools, however: silhouette, fabric, colour, detailing, construction, finish etc. I realise this is an old thread and you may no longer be watching but if you'd like to post more photos we can help date.
Nicole