Snape's job at Hogwarts

[khinterberg]

No: HPFGUIDX 111107

In OotP, during the Potions class in which Umbridge supervises Snape,
he says that he has held his job there for 14 years, which means he
got the post the year baby Harry's life changed.  What are ideas of
why he got the job then? We also know he first applied for DADA, but
didn't get that.  So were both positions open that year?  

khinterberg, who wants to know more about Snape