The date that you first used your trademark could be, but probably is not, the correct date to use as the first use in commerce date in a federal trademark application. Why? Because in many cases the first use of the trademark was on a website or marketing material before an actual sale or shipment of the goods or rendering of the service.
First Use in Commerce
For each class of goods or services in a federal trademark, you must declare when you first used the mark in commerce for those goods or services. Use in commerce has a specific meaning under the law. It does not mean simply using the mark. The use must also be combined with a sale of goods or a rendering of the services.