Form 104 and any tax payment owed are due April 15. Revenue Online will accept returns as timely filed until midnight. Returns that are mailed must be postmarked by April 15th. An automatic extension to file is granted until October 15th, but there is no extension to pay.
Mail your return and, if applicable, your tax payment to:
Colorado Department of Revenue
Denver, CO 80261-0008
The envelope must be postmarked by April 15.
Include the payment voucher with your payment.