When to file Permesso di Soggiorno

Hello,

I have just arrived in Italy under a student Visa type C+D multiple entry from the USA.  I arrived on Jan. 6th, but my visa begins on January 13th.  I know I have 8 days to declare my presence at the Questura and apply for the Permesso di Soggiorno. 

My question is... do I have 8 days from the date I arrived through Italian customs (Jan. 6th) or 8 days from the date listed on my visa (Jan. 13th). 

Thank you!

8 days after you arrived. when you had your entry stamp from the immigration at the airport.