Numbers 33:3 very plainly states that the next daylight period after Passover was the 15th, not the 14th, so the JW's are a full 24 hours early in their observance.
Insight Book Volume 2 Page 580, par 3
“Since the Jews reckoned the day as starting after sundown and ending the next day at sundown, Nisan 14 would begin after sundown. It would be in the evening after Nisan 13 concluded that the Passover would be observed. Since the Bible definitely states that Christ is the Passover sacrifice (1Co 5:7) and that he observed the Passover meal the evening before he was put to death, the date of his death would be Nisan 14, not Nisan 15…”