Nisan 1 starts when the new moon nearest the spring equinox becomes visible at sunset in Jerusalem. Counting 14 days from that event, is always Nisan 14.
To illustrate how the above is false due to Jewish leap years:
Last year was a Jewish leap year, wherein we had to add an additional month at end of the year. The extra month is called Adar I, Adar Rishon, or Adar Aleph. And then the regular month of Adar is called Adar II, Adar Sheni, or Adar Bet (I know, it's confusing).
This places everything around 29/30 days later.
So if we use Duran's calculation, what happens?
In 2024, the new moon that was closest to the spring equinox in Jerusalem was on March 10 at 9:02 AM. The spring equinox was on March 20 at 3:08 AM.
To determine the beginning of Nisan 1 in Jerusalem, add two hours and twenty-one minutes to the time of the new moon on March 10. This would make the beginning of Nisan 1 at 11:23 AM on March 10 and Nisan 14 on March 24.
That was also the date when Jehovah's Witnesses observed the Memorial.
But, alas, last year was a leap year on the Jewish calendar. You have to have leap years because it is a lunar calendar--otherwise it goes out of whack. It has always been that way.
That calculation was wrong.
Last year, Nisan 14 landed on April 21, 2024. Why do I know? Because Passover started on Monday, April 22. That was Nisan 15 of last year.
It was a leap year.
Both the Jehovah's Witnesses and Duran were wrong. They did not follow the way the Sanhedrin or the modern or the ancient Jews did it.
They forget the Jewish leap year.
Thus you have to use the Jewish calendar system and pay attention to when a leap year goes in and when it does not. It does not rely on the Spring Equinox or when anything is "visible in the sky."