It just depends on how you define the sales period. If you're limited solely month to month, maybe, but that's not a very helpful.
It's also worth pointing out there are multiple definitions of market share, and irlju just chose the one that helps make his polemical point.
Market share - Wikipedia, the free encyclopedia
""Market share is the percentage of a market (defined in terms of either units or revenue) accounted for by a specific entity." In a survey of nearly 200 senior marketing managers, 67 percent responded that they found the "dollar market share" metric very useful, while 61% found "unit market share" very useful."
The pretty common sense meaning--
What is market share? definition and meaning
The percentage of the total sales of a given type of product or service that are attributable to a given company.