Yes you are right Juandefiero, I have been checking out that website ...its a jewish leap year and they add an extra month. Its the 15th that must fall on or after the equinox..thats where they got that bit of mis-information I read.
However, the Watchtower is WRONG that modern Jews dont celebrate on the 14th like Jesus did...they do..its essentially two celebrations on the 14th AND the 15th..the following explains why....
The Passover lamb was SLAIN "AT EVEN" (Lev. 23:5) -- Josephus shows it was essentially between 3-5 P.M. of Nisan 14. Then it had to be taken home from the Temple, put on a spit, and cooked -- roasted -- for hours. Therefore, the Passover meal was EATEN, and CELEBRATED, at night on Nisan 15. The Jews have always known this.
When Scripture says "they kept the Passover on the fourteenth day of the first month at evening," (Num.9:5), it is referring to the fact that the lambs were SLAIN at that time! But since you cannot slay and eat at the same time (unless you ate it raw!), you must add to that time of slaying the time needed for preparation, cooking, and eating, which must of necessity be several hours later -- or on the 15th.