D.C. Mun. Regs. tit. 9, r. 9-148
If a taxpayer files a District income tax return for a calendar year on or before January 15th of the succeeding calendar year (or if the taxpayer is on a fiscal year basis, on or before the 15th day of the first month immediately following the close of that fiscal year) and pays in full the amount of tax shown on the return as payable, that return shall also be considered a declaration for the taxable year if the taxpayer first met the requirement with respect to filing declaration after September 1st of the taxable year (or the 1st day of the ninth month of the taxable year for fiscal year taxpayers).
If the tax shown on the return differs from the estimated tax shown in the previously filed declaration, the return shall be considered as an amended declaration the filing of which before the 15th day of the first month following the close of the taxable year is permitted.
If a taxpayer files on or before September 15th a timely declaration for that year and after that (on or before January 15th of the succeeding taxable year or corresponding date in the case of a taxpayer on a fiscal year basis) files a return for that year, and pays at the time of filing the tax shown by the return to be payable, the return shall be treated as an amended declaration timely filed.
For the purposes of this section a taxpayer may file a return, Form D-40, on or before the 15th day of the first month following the close of the taxable year even though he or she has not been furnished Form D-2 by his or her employer.
When filing a return in accordance with § 148.4, the taxpayer shall compute, as accurately as possible, his or her wages for the year and the tax withheld for which he or she is entitled to a credit; and shall report those wages and tax on the return, Form D-40, together with all other pertinent information necessary to the determination of tax liability for that year.
D.C. Mun. Regs. tit. 9, r. 9-148