You are the accountant for Klemmer Corporation. At the company's year end, December 31, 2020, you discover there is an amount of $30,000 that has been earned by Klemmer but not yet billed to its customers by the year end. Laura Klemmer, the owner, tells you not to bill the customers as it is company policy not to bill customers until February 2020, well after the Christmas holidays. Klemmer has sales staff that are paid a bonus at year end on sales revenue billed. -What accounting principle has been violated by not billing customers for the year ended December 31, 2016? Explain.