"Date of First Use in Commerce" is a term that indicates a date when a mark was first used in commerce in connection to given goods and services in the US market specifically. It is not the date when the mark was used in a different market - such date is referred to as "The Date of First Use Anywhere".
"Date of First Use in Commerce" is a very important piece of information that has to be included in the US trademark applications that are filed with "Use in commerce (1a)". A specimen must be attached to the application as proof that the mark was used "bona fide" in the US market. It is recommended to keep records and documents related to the date and location your goods/services were first sold in the US market. You can provide such documentation to the USPTO when filing your application.