Daf Yomi - Megillah 20 - It's Night after Plag Hamincha

The Gemora states that it is not regarded as night until the stars come out.

It is brought in the name of the Imrei Emes (Eretz Tzvi (25/26) that if one davens Maariv on Erev Rosh Chodesh (the day before Rosh Chodesh) from plag hamincha (one hour and fifteen minutes before sunset) and on, he should not recite yaaaleh v'yovo for Rosh Chodesh in Shemoneh Esrei.The reason provided is based on a Magen Avraham (419), who cites from the Shalah that one cannot add additional time to Rosh Chodesh like he can to Shabbos and Yom Tov.

The Mishna Berura (693:4) rules regarding one who davens Maariv after plag hamincha before Purim; he should recite al hanisim in Shemoneh Esrei. The Sha'ar Hatziyon comments that this is based on the ruling that one may daven Maariv for the next day after plag hamincha and there are no poskim who rule that one should not recite yaaleh v'yovo when he is davening Maariv early before Rosh Chodesh. One is allowed to recite havdalah after plag hamincha if he already davened Maariv. Mishna Berura concludes that it is evident from here that from plag hamincha and on; it is regarded as night in respect to davening.