If you're self-employed, an S-corp owner, or running a partnership, September 15 is your Q3 estimated tax deadline. It sneaks up on a lot of contractors because it doesn't line up with the calendar the way most people expect taxes to work.

Estimated taxes exist because the IRS wants its money as you earn it, not just once a year in April. If you're a W-2 employee, that happens automatically through withholding. If you own your HVAC business, nobody's withholding anything for you, which means you're responsible for sending in your own quarterly payments based on what you expect to owe for the year.

Miss a payment, or underpay significantly, and you're looking at penalties on top of whatever you already owe. The math isn't forgiving. The IRS calculates the underpayment penalty based on how much you owed and how late the payment was, and it compounds the longer it goes unpaid.

The bigger issue we see isn't contractors forgetting the date. It's contractors who don't actually know what number to send in, because their books aren't current enough to calculate it accurately. If you don't know your year-to-date net income, you're either guessing low and setting yourself up for a penalty, or guessing high and tying up cash you need for payroll and parts.

This is exactly why we push our HVAC clients to stay current on bookkeeping month to month instead of scrambling every quarter. When your P&L is accurate as of today, calculating your Q3 payment takes minutes instead of a weekend of digging through bank statements.

If you're not sure what your Q3 payment should look like, or you're not sure you've been setting aside enough throughout the year, now is the time to find out, not after the 15th has already passed.

Jeremy Brewer is the founder of 911 Bookkeepers LLC in Baton Rouge, Louisiana. He worked in the field as an HVAC tech before building books for the trades, and he serves as a licensed paramedic in EMS. He is a Xero Certified Advisor.