Will the person who intends to fast ‘Ashoora during the day be rewarded for his fasting?


I know of the virtue of fasting the day of ‘Ashoora, and that it is expiation for the year that came before. But because we use the Gregorian calendar I did not know when ‘Ashoora is until the morning of the day itself. I had not eaten anything, so I intended to fast. Is my fast valid, and will I get the reward of this day and receive expiation for the previous year?.