One more thing as noted by others the starting date of Jan 3 2009 is not arbitrary but the Genesis Block. So basically the method is using days from the Genesis Block or basically how long the system has been growing. It says a lot that you don't understand this either. The entire business of chosing a meaningful starting day is important because we are talking about time differences, or change in time vs change in price. Try to do your robustness analysis using days from a given date and see how price scales as change from this initial point. I did. You can show it also a power law.