Utilize your financial knowledge, give your answer or opinion to the input question or subject . Answer format is not limited.

Why are estimated taxes due “early” for the 2nd and 3rd quarters only?
There are too many nuances to the question asked to explore fully but here are a few points to keep in mind. If you are a cash-basis taxpayer (most individuals are), then you are not required to pay taxes on the money that has been billed but not received as yet.  If you operate on an accrual basis, then the income accrues to you the day you perform the service and not on the day you bill the client. You can make four equal payments of estimated tax on the due dates, and if these (together with any income tax withholding from wage-paying jobs) are at least 90% of your tax liability for that year, then you owe no penalties for underpayment of tax regardless of how your income varied over the year. If your income does vary considerably over the year (even for people who only have wages but who invest in mutual funds, the income can  vary quite a bit since mutual funds typically declare dividends and capital gains in December), then you can pay different amounts in each quarterly installment of estimated tax. This is called the annualization method (a part of Form 2210 that is best avoided unless you really need to use it).  Your annualized income for the payment due on June 15 is 2.4 = 12/5 times your taxable income through May 31. Thus, on Form 2210, you are allowed to assume that your average monthly taxable income  through May 31 will continue for the rest of the year. You then compute the tax due on that annualized income and you are supposed to have paid at least 45% of that amount by June 15. Similarly for September 15 for which you look at income through August 31, you use a multiplier of 1.5 = 12/8 and need to pay 67.5% of the tax on the annualized income, and so on.  If you miscalculate these numbers and pay too little tax in any installment, then you owe penalties for that quarter.  Most people find that guesstimating the tax due for the entire year and paying it in equal installments is simpler than keeping track of nuances of the annualized method. Even simpler is to pay 100% of last year's tax in four equal installments (110% for high earners) and then no penalty is due at all. If your business is really taking off and your income is going to be substantially higher in one year, then this 100%/110% of last year's tax deal could allow you to postpone a significant chunk of your tax bill till April 15.