I'm not a financial guy, but the math seems right. From an XL standpoint, I'd accomplish this by having with today's date (either manually typed as a form of date stamping, or with the TODAY() function). Then, if date is in A1, and sales are in A2, formula is:
=A2/(A1-DATE(YEAR(A1),1,1))*365
Other small deviation I might think of including would be to switch to using work days rather than calendar days. Would only be a small variation, but might make a difference, depending on your industry.