Do the Minhagim of the 9 days start at shkia or tzeis?
The customs of the Nine Days begin from Shkiya.
The Magen Avraham (551:26) and Eliya Rabba (19) write that the customs begin from Shkiya.
Although they are “only” customs, so that there should seemingly be room to be lenient even in the time of Beis Hashmashos, the Pri Megadim implies that this is how the custom was originally enacted: From while it is still daytime before Rosh Chodesh. This is likewise given as an explanation for the Magen Avraham by the Chedvas Yehoshua.
Therefore, the customs should be accepted from Shkiya.