They use a different formula to calculate Nisan 14, which may or may not coincide with the official calendar. To me, "Nisan" is not even a month--officially, we have January, February, March, April, May, June, July, August, September, October, November, and December (in the English language). Nisan is not even among those. And those months we do have last 30 or 31 days each (except February, which is either 28 or 29), not all months having 28 days.
Next, I suppose they are going to try putting everything in the hebrew language. Using hebrew letters, to boot. Anyone else try putting appointments in hebrew, or using Nisan as a month? It doesn't work so well. And besides, are they claiming to be Christian? Then why are they pretending to be Israelites? (Ancient Israelites, to boot. Even if they are in Israel, they are not going to find the Nisan calendar in use, and they are not going to get far just speaking hebrew.)