According to the United States internal revenue service, federal estimated tax payments are due on this schedule:

• April 15
• June 15
• September 15
• January 15 (of the next year)

June is only two months after April. I would expect estimated tax payments to be evenly spaced three months apart. Why are estimated taxes for the second and third quarter due a month earlier than I would expect? (I would expect July and October.)

I usually bill my clients at the end of the month. In June, taxes are due for the 3rd quarter, but I haven't even received the June payments yet. Do I owe taxes at that time on the money that I expect to receive that quarter, but which hasn't actually come in yet?