The dates for the births of the children are, not unusually, approximations: http://fmg.ac/Projects/MedLands/ENGLISH%20NOBILITY%20MEDIEVAL.htm#H...
The date of Hugh’s birth is likewise an approximation.
But his is based on the fact that his father’s first wife must have been dead before he married again, to Hugh’s mother.
His children’s, as far as I can tell, are based on the dates of his own two marriages, and events in their lives.
The dates in Geni are about the same as the ones in Wikipedia; they are commonly used.