Night Of Purim

Rav Amram Gaon (cited in Haghos Mimoni Tefillah Chapter 2 #6) holds that we don't recite al hanissim at night by maariv since it is only said after the Megillah is read. Why? The Divrei Chaim blog in 'Assorted ideas on zachor and Purim' has one explanation. Aa a הוספה I see that the approach of tying the על הניסים to the event of the miracle of the day is linked by Rav Tzvi Ryzman to a chakirah if al hanissim is a halacha in the day of Purim or a law in tefillah to mention מעין המאורע. If it is a law in Purim itself then one can suggest the miracle of the day must be said first but if it is a law in מעין המאורע as part of tefillah (as indicated by Rambam who puts mentioning al hanissim in Laws of Tefillah) then it would not make sense to require Megillah to be read first.    

I would like to share another explanation. 

We find that the main fulfillment of Purim is in the daytime. The mitzvot of Purim can be fulfilled only in the daytime. In addition, in the sugya of a person who moves from a city that reads on the 14th to one that reads to the 15th the Rosh says what determines when a person's obligation in Megillah is where he is on the day of Purim. We see it is the daytime that creates establishes the obligation of the person. Why is the daytime that determines the obligation if we read Megillah at night as well? What we see is that it is the day of Purim that carries the kedushas hayom of Purim. The night is just an obligation of reading of the Megillah but it does not establish the essence of Purim. In light of this Rav Amram holds that one can't say al hanissim at night. Only after the Megillah is read and one has declared the miracle can one say al hanissim (I saw this idea cited in the name of Rav Soloveitchik in Harrerey Kedem siman 13 but I don't have the sefer to check, see also ביסוד דין קריאת המגילה ביום ובלילה ובגדר הלילה דפורים) 

The Mechabar (692:4) says that in a שעת הדחק one can read the Megillah already from פלג המנחה. The Pri Chadash and Gra disagree. In light of this explanation it may be easier to hear how one can be yotzei from plag for it is not a nighttime obligation, it is just an obligation to hear the Megillah during the time connected to Purim.