Ms. Marsh filed her 20X0 individual income tax return on February 15, 20X1. All her tax was paid during the year through withholding. The return was due on April 15, 20X1. During January 20X2, she discovered that she had not taken a properly substantiated charitable contribution that would have reduced her total tax by $250 on her 20X0 tax return. By what date must she file her amended return to claim a refund of the tax paid?