I've changed Severus' birth year from 1960 to 1959 according to new information I compiled. This is a huge puzzle for everyone so I've been compiling ALL possible information pertaining to Snape's birthday.  It should reveal the birth year of Lupin and James for sure.  I've gone where no one has before, apparently, so it may change lots of dates in the end because the HP Lexicon always said the most probable date was 1960. However, after analysing all the facts we were presented (canon and official only), I've made many calculations and ended up changing the year of events from 1959 up to 1980 when Harry was born.
 

Facts from all 6 books and Rowling

1. The real problem is knowing in what year they all began! If we had this, it would be easy to extrapolate the necessary information. But we don't. Here is what we have however:
    
2. Students can only enter Hogwarts if they are at least 11years old on Sept. 1st. Even if you were born in September like Hermione or on Sept. 2nd, you could not enter.
    
3. Hogwarts 5th Year students must be at least 15 years old when they come to school on Sept. 1st
    
4. Therefore, Snape's birthday being in January, he was 11 and 7 months when he got into Hogwarts.
   
    Snape's AGE

	Hogwarts Years	Start of school Year (September 1st)	End of school year (mid-June)
OWLs NEWTs	1st Year: 2nd Year: 3rd Year: 4th Year: 5th Year: 6th Year: 7th Year	11 years and 7months 12 years and 7months 13 years and 7months 14 years and 7months 15 years and 7months 16 years and 7months 17 years and 7months	12 years and 5 months 13 years and 5 months 14 years and 5 months 15 years and 5 months 16 years and 5 months 17 years and 5 months 18 years and 5 months

5. OWLs are in 5th Year.  In the first two weeks of June.
    
6. Snape and Lupin were at least 16 when they sat their OWLs.
    
7. The Marauders, Lily and Snape were all in the same year. Only Lupin and Snape have specific birthdays.
    
8. Hogwarts students who started in 1970 were born in-between Sept.2nd 1958 and Sept. 1st 1959.
   Students who started in 1971 were born in-between Sept.2nd 1959 and Sept. 1st 1960.
   James still being 15 near the end of his 5th Year proves he had to be born in the exact same year as Snape and Lupin or else he would have been almost 16 or would have entered Hogwarts a year later. It is a correlation: if Snape was born in 1959, so was Lupin and James. If Snape was born in 1960, so were Lupin and James. Sirius could also follow the same pattern (see calculations)
    
9. Rowling mentioned on her site that Lupin and Sirius are the same age but have different maturity. So their birthdays must not be so far apart (Spring or early Summer).
    
10. (Snape's Worst Memory ) Snape was described as being fifteen or sixteen, around Harry's own age. It makes sense because he was 16 and 5months.
     
11. James was fifteen at that moment but Harry was able to tell right away without hesitation. So maybe Snape looked a bit older. If so, and because of Hogwarts rules, then James's birthday must be sometime during the summer because he was still 15 while sitting his OWLs! He is younger than Severus but born the same year.
     
12. Sirius said of James that "of course he was a bit of an idiot" and "we all were idiots".  This suggests that they were maybe close in age as well. Since Lupin was born in March, we can assume Sirius comes next and/or around the same time as James in summer.
     
13. Release of Goblet of Fire in March:  J.K. Rowling said Snape is about 35 - 6.
• She could mean that he was 35 or 36 at the beginning of the book, at the end of it or all through that school year.
• But since she said "is" and not "was", we can assume she referred to the end of the book as in "up to now in the series, Snape's 35 - 6" OR the whole book as in "In Goblet of Fire, Snape is 35 - 6" 
   
14.  Goblet of Fire happened from school year Sept. 1994 to June 1995
     
15. Rowling mentioned on her site that Sirius was about 22 when he was sent directly to jail after being refused a trial on November 1st, 1981. So 'about' may mean 21, 22, 22 and a couple of months, or 23. However, since she said 22 and not 21 or 22/22 or 23, we can assume 22 is the best age and that the "about" meant that he had 22 and a couple of months... (See my calculations here)
     
16. Snape was hired on Sept. 1st 1981. Umbridge inspects his classroom on October 7th 1995 and mentions he's been teaching for 14 years. This is in  accordance with the fatidic night of Nov. 1st 1981.

Testing and sorting out this puzzle

Years start in September.
A year represents the 10months of school plus July and August at the end of each year
This is based on the year of the Goblet of Fire (1994-1995)
Snape must enter Hogwarts at 11years and 7months
Snape must end his first year at 12 years and 5 months
Sirius arrested at about 22 on November 1st, 1981. 13 years before the Goblet of Fire.

- Conclusions -

Considering ...

• that when Rowling said Snape 'is' 35 - 6 after publishing Goblet of Fire, she most probably meant it as in "up to this date in the series"

Then Snape was most likely born in 1959 OR 1960.
Then Snape would have become a teacher at Hogwarts at 22 or 21.

However, considering ....

• that Sirius was about 22 (and some months) on Nov. 1st 1981 (see calculations)
• that Lupin was the same age as Sirius
• that Lupin was born only two months after Snape (making those three close in age)
• that Lupin and Snape were also X-years  and some months old
• that they were all in the same school year
• that you need to be 16 in your 6th Year and 17 in your 7th at Hogwarts

Then Snape was most likely born in 1959.
Snape would most likely have become a teacher at 22.
He entered Hogwarts in 1970 and graduated in June 1977.
Three years later, Harry Potter was born in late July 1980.
Halloween 1981 - Fall of the Dark Lord giving Snape at least four years out of Hogwarts
and as a possible Death Eater. Maybe more if he joined (after he was of age in January 76).

Wallpaper

 I've used the astrological sign attributed to January 9th because I believe that Rowling may be someone who would purposely look up such "muggle magic" before making a choice.  I must say that, though I've seen somewhere that most people believed he was a Scorpio, that this sign suits him very well.  See this website here that I found while looking up information on Capricorns yesterday, and you'll know what I mean. (It's a good site I reckon since there was no advertising on it! Which is quite rare!)

The VS shaped symbol or sometimes written like this '6 over the Internet is the sign of Capricorns which is why I used it for this wallpaper.  I think it suits Snape very well, too.

I took the round Capricornus icon on this webpage where the whole set is featured. Please visit!

I also found this funny picture of a Capricorn which might have inspired Rowling to decide upon Snape's birthday... What do you think? ; ) Coincidence?!