Machtzis Hashekel After Purim

As mentioned before, there is an obligation to give a replica "machtzis hashekel" on the day before Purim (רמ"א סי' תרצ"ד ס"א). Why specifically then was explained here.
What if someone forgot to give it and Purim has already passed - can they still make it up afterwards?
The source of the machtzis hashekel is the half shekel that every Jew would give to the Beis Hamikdash. The time the Mishnah gives for this is the month of Adar. [See Rambam (רמב"ם הל' שקלים פ"א ה"ט) for a detailed description of how the process spanned the entire month.] Thus, if one has the whole month to remember this idea, even after Purim.
Not only that, but they can fulfill their obligation to give machtzis hashekel the entire year until Rosh Chodesh Nissan of the following year. Thus, if one forgot to give machtzis hashekel before Purim of 5771, they can give it until Rosh Chodesh Nissan of 5772.
שו"ת אבני ישפה (פיינהנדלר) ח"א סי' קל"ג
ועי' אורחות חיים (כהנא) סי' תרצ"ד סק"ב
See here