taxes: put assignment question

[IHateMyJob2004]

Lets say I sold a put option on 1/1/2007

It was assigned on 5/5/2007

For tax reporting purposes, is the purchase date for the assigned shares 1/1/2007 or 5/5/2007?

 

[IHateMyJob2004]

I think the holding period starts the day of assignment (5/5/2007). Can someone verify this?