I think Pesach is April 11 [starting the night of April 10] since there is no Beit Din to sanctify the new moon, and Hillel II never set up the present day calendar. It is the calendar of Meton from Athens not from Halacha LeMoshe MiSinai. It was adopted in the middle of the geonic period. Thus it makes sense to go by the Gemara in Sanhedrin and the Mishna in Rosh Hashanah that when the Beit Din on earth do not sanctify it, the new moon is sanctified from heaven.
The idea of April 11 is the the new moon was March 28 Universal Time in Greenwich at night time 2:57 AM. That is the night after Monday. That is the new moon was Monday night after midnight. So Tuesday March 28 was Rosh Hodesh. So next Monday night is the 15th of Nisan.
The idea of April 11 is the the new moon was March 28 Universal Time in Greenwich at night time 2:57 AM. That is the night after Monday. That is the new moon was Monday night after midnight. So Tuesday March 28 was Rosh Hodesh. So next Monday night is the 15th of Nisan.
