Snape thoughts

[houyhnhnm102]

No: HPFGUIDX 136863

Marianne S.:
If I understand the timeline correctly, Snape was working for
Voldemort at the time he first applied for a teaching post at
Hogwarts. It is unclear to me if he started teaching that year or the
next, after the fall of Voldemort. 

houyhnhnm:

During Dolores Umbridge's "walk through" of Snape's classroom, he
tells her he as been teaching at Hogwart's for 14 years.

"Now...how long have you been teaching at Hogwarts?" she asked, her
quill poised over her clipboard.

"Fourteen years," Snape replied.  His expression was unfathomable.
(OotP, AR, p. 363)

This means that he would have been hired when Harry was around a year
old, which is consistant with what we already know and take for
granted, that Snape went to Dumbledore and offered to turn his coat a
few months *before* the Potters were killed.

A few pages later, when Professor Trelawney loses it in class, after
getting the results of her inspection back, she says:

"I say nothing," she choked, "of sixteen years devoted service .... 
It has passed, apparently unnoticed....But I shall not be insulted,
no, I shall not!" (OotP, AE, p. 366)

This means that the prophecy must have been made at least a full year
before Harry was born, which I had not realized before.  And it means
that two years elapsed between the time Snape overheard the prophecy
and the time he began teaching at Hogwarts.