They do pay interest that accumulates some time after the filing deadline. This is probably just a practical matter, because accounting for interest during the

They do pay interest that accumulates some time after the filing deadline. This is probably just a practical matter, because accounting for interest during the tax year would be complex/impossible because you would need to know when the various amounts of withholding exceeded the amount owed. I'm not even sure there is a single consistent way of doing this.