I simply filed for an extension because my tax due was much greater than we expected. Our employer screwed up my withholding, after I revised it to reduce how much I pay every year. Their negligence resulted in my withholding being insufficient, increasing how much more I had to pay for 2018 Therefore we don't have the money to pay it right now & have to get with a financial adviser to see if I have to borrow against my TSP retirement fund. We're pretty angry with our employer right now.
