Tax season runs on specific dates, and missing one by even a day can mean penalties. According to the Taxpayer Advocate Service’s 2026 tax calendar, January 26 marked the official start of the 2026 filing season — the first day the IRS began accepting and processing returns for the prior tax year.

The key dates worth marking

Beyond the season opener, a handful of dates carry real deadlines: February 2 is the due date for employers to issue W-2 and 1099 forms; April 15 is both the standard deadline for individual returns and the deadline for first-quarter estimated tax payments for the self-employed; and October 15 is the final deadline for anyone who filed for an extension. Partnerships and S-corporations work on a separate calendar-year deadline of March 16. Self-employed filers also have a fourth-quarter estimated payment due January 15, ahead of the main season entirely.

Why „90 days“ and „30 days“ style calculations trip people up

A surprising number of tax-deadline mistakes come down to simple date-math errors, not confusion about the rules themselves — miscounting how many days remain until a deadline, or miscalculating a quarter’s estimated-payment window, especially across a month boundary. Manual day-counting is exactly the kind of task that’s easy to get subtly wrong, particularly when a deadline falls near a weekend or federal holiday and shifts to the next business day.

Count the days precisely, not approximately

Whether you’re figuring out how many days remain until April 15, checking the exact gap between two estimated-payment dates, or working out a deadline that’s „90 days from“ a specific filing date, our date calculator handles the exact day-count automatically, including leap years and month-length differences that manual counting often gets wrong.