is the world going to end May 21?
No.
Their entire premise is based on the statement that, "Years ago we learned from the Bible that the flood occurred in the year 4990 B.C."
The commonly accepted date for the flood is ca. 4350 years ago, so all of their numbers are wrong from the start.
And the flood is a local tribal myth anyway.
(Yawn.)
Religious belief does not constitute scientific evidence, nor does it convey scientific knowledge.