They do not follow the modern Jewish calendar in determining the memorial.
This from aQFR in 1977
” Appropriately, the date for the Memorial celebration is arrived at as the Jews back then determined the date for the Passover. They began the month of Nisan when they could first see the new moon in the spring nearest the equinox. Passover came fourteen days later.—Isa. 66:23; Ex. 12:2, 6.a
Jehovah’s Witnesses now follow this ancient pattern in determining the Memorial date. Please note that the first thing that needs to be established is when the new moon nearest the spring equinox (about March 21) will be visible in Jerusalem. “
From what I read online the new moon UK , is March 10. So 14 days on is March 24 . A snappy phrase I remember is “ The first full moon after the Spring equinox “ . That also gives their date…
Basically they are saying, never mind the Jews , we think we are right.