Today marks the second deadline for 2024 tax year estimated payments. The IRS calls them quarterly estimated taxes, but they aren't due on traditional quarterly dates nor equal to three months of payments. They represent a quarterly portion of your estimated tax liability paid at irregular intervals. The first payment is due at 3.5 months, the second at 5.5 months, the third at 8.5 months, and the fourth at 12.5 months.