Timeframe

• Federal and state taxes are due by April 15 if you use a calendar where the year ends at December 31. If you mail in your tax forms, they must be postmarked by the 15th, but will not be considered late if they arrive later, due to the postal process.
  
  

Considerations

• Businesses that use a fiscal year -- defined as a year that ends at the end of any month other than December -- should file their taxes by the 15th day of the fourth month after fiscal year close. For example, if a business's fiscal year ended in March, taxes would be filed on or before July 15. Also, if April 15 falls on a weekend, taxes are due on the next business day. This happened in 2011, with taxes due April 18.
  
  

Exceptions

• Military personnel serving overseas don't have to abide by the April 15 deadline. They can instead complete tax forms up to 180 days after they leave the combat zone. Internal Revenue Service Publication 3 details tax deadlines and information for those serving in the military. American citizens living overseas, who also work primarily overseas, receive a two-month extension. They must file by June 15. IRS Publication 54 is a tax guide for those living abroad.
  
  

Extensions

• If you can't complete your tax form on time, you may request an extension using Form 4868. While anyone can request an extension, you may be charged a late fee if you don't have "reasonable cause" for needing an extension. When you file the form, write a statement explaining why you could not file on time; if the IRS agent deems the reason appropriate, you won't be charged a late fee. You may file Form 4868 electronically, by mail or by paying part of the tax due through an outside provider listed on the tax form.