When does a bussiness start? Startup costs and expenditures

My wife created an EIN and a DBA for her business (in California) in 2023. She has had expenses related to starting the business before the date she created an EIN and after the date she created it. Thus far she hasn't yet started selling her product (no income yet), but anticipates doing so in 2024.

What is considered the official start date of her Business? Has it already started or not because she hasn't yet sold any product? Are these expenses (materials, ect.) considered to be "startup costs" or expenditures or both depending on the date that she filed for an EIN or DBA?

Thanks so much!

Stephen