When did Snape invent his curses?

[lapland874]

No: HPFGUIDX 133323

Snape had apparently invented several curses as evidenced by the potion 
book which Harry used all year.  However one curse was actually used 
the previous year.  When James turned Snape upside down was on the day 
of his OWL's exam.  The advanced potions book was not used by Snape 
until the following year.  Unless he was already using it?  So did 
Snape invent that curse/jinx or did he just figure it out and then 
write it in the book?  Obviously some of them where truely his, but 
maybe not all of them.


Lapland874