It is wise not to get too caught up with calculating Bible dates. They were written to show lineage (who descended from whom), and they can occasionally skip a generation or two, or three.

The issue is that ‘begat’ or ‘became the father of’ does not always mean the same thing. It can mean that someone ‘became the ancestor of’ someone.

This article on Ancient & Biblical Chronology & Genealogies has a good explanation. If you want to save a bit of time, scroll down to the section on Biblical Genealogies.