I'm a manufacture that builds doors. when is my start date from when there deposit clears the bank or from shop drawing approval or when i start on building. we are always 2-3 weeks from starting any job after receiving a deposit because we are always finishing another job as i have this one line up to jump on once we complete last job.
If your doors are custom -- meaning they good not be easily used on another project -- then the start of construction or first furnishing of labor, material, or services, for the purposes of a notice to owner, is the date you start performing work. You have 45 days from that date to serve your notice to owner. But, you have from the day you sign the contract to that 45th day to serve the NTO, so I would do so earlier rather than later.