Hank, a calendar-year taxpayer, uses the cash method of accounting for his sole proprietorship. In late December, he performed $20,000 of legal services for a client. Hank typically requires his clients to pay his bills immediately upon receipt. Assume his marginal tax rate is 32 percent this year and will be 37 percent next year, and that he can earn an after-tax rate of return of 12 percent on his investments. Should Hank send his client the bill in December or January?