Despite the IRS referring to these payments as quarterly estimated taxes, the due dates don’t necessarily fall within “quarters” nor do they each represent three months of tax payments. They represent an equal quarterly share of your estimated income tax liability paid at uneven intervals. The first payment occurs 3 and a half months into the year. The second payment is five and a half months; the third payment is eight and a half months, and the fourth payment is due 12 and a half months after the year starts.