The Mitzva of Mishloach Manos is on the day of Purim. If you send someone Mishloach Manos before Purim and they receive it on Purim are you yotzei your mitzvah of Mishloach Manos since you did not
actually give it on Purim?
The Bais Ovi (4:61) says that starting a mitzvah is considered doing the mitzvah, even if you cannot complete it because its time has not yet come. He brings two examples as proof. First by Bedikas Chometz we make a Bracha before the bedika at night even though the mitzvah is the burning, which is not until the next morning.
The second example is by the mitzvah of Kisui HaDam, covering the blood of a bird that you slaughter with dirt. The halacha is that if you “Shecht” a bird on a boat and do not have dirt to cover the blood, you soak up some blood in an article of clothing and you make a bracha at that time even though you have not yet done the mitzvah. When you reach dry land you then wash it out and cover the blood with dirt.
Similarly he says, you may do the sending of the Mishloach Manos before Purim and if it arrives on Purim you have done your mitzvah, even if your actions took place before the time of Mishloach Manos.