Business Issues: What is the Legally Correct Year of Establishment for a Business

If a person taught courses/tutoring as an independent contractor before forming a LLC as the business grew bigger the following year, what is the legally correct establishment year to use in the company's marketing? Many companies put something like "Est. Year" or "Since YYYY" next to their logo, so in this case, say if the person began tutoring independently in 1997 and the LLC was established in 1998, would it be legally correct to state "Since 1997" or Since 1998"?

Business Issues: What is the Legally Correct Year of Establishment for a Business