I suspect the long lead items on the definite commitment to build the new Chinese carrier with specific long lead items were made around 2008. That was when China first made specific public commentaries about the need for a nation like China to build carriers. That was really not an opinion piece but really meant as an oblique announcement that carrier building is underway.
Based on other information, the formal contract to built the hull was probably not issued at the time, and two ship yards were still in competition for the contract.
The basic hull design was no doubt complete by 2008, but the detailed engineering design suitable for cutting steel was probably not completed until the shipyard was selected and could participate in finalizing the last details needed by the fabrication technique. I think the yard was selected around 2010, the blue prints finished a year later, and
The fabrication of the first hull component probably started around beginning of 2012.