Adrian Buckman shows what happens if you start fiddling with SQL Agent tables:
Some time ago I came across a strange issue where I found a number of duplicated SQL Agent jobs, the odd thing is SQL will not allow you to have more than one agent job with the same name – they need to be unique.
This got me scratching my head a little at first, so I started out with some basic checks of the msdb tables.
This is example #5008 of just how poor the SQL Agent database design is. Example #1 is the absurd date-time notation.