Taxes are collected on a quarterly basis and are due on February 1, May 1, August 1 and November 1 of each year.  Your quarterly payment covers three months--the month before they are due, the month they are due and the month after they are due.  You are given a 10-day grace period after the due date before interest is payable.  For example, the payment due February 1 must be paid by February 10.  Any payment received February 11 will be charged interest back to February 1.