Mulan....Actually, they don't say that. It is the new moon nearest to the spring equinox, not necessarily after the equinox. That means in some years, the full moon precedes the equinox. This is one such year.
*** w59 3/15 p. 187 Why Celebrate the Lord?s Supper? ***
Nisan is the first month of the Jewish lunar year and begins with the visible new moon nearest the spring equinox.
*** w60 4/1 p. 198 The Anniversary That Must Be Kept by Christians ***
Yes, according to the lunar calendar that God gave the Israelites, the year began in the spring with the new moon nearest the spring equinox; the equinox usually falling on March 21, the first day of spring. Nisan or Abib, the first month, began with the visible new moon nearest that date.
*** rs p. 269 par. 2 Memorial (Lord?s Evening Meal) ***
The beginning of the month of Nisan was the sunset after the new moon nearest the spring equinox became visible in Jerusalem.