How to determine actual device activiation date?

Question

My Verizon plan has five phone lines.  Often we will grab an available two year renewals from one line and use it for another line.  My son noticed that his line line has a renewal coming up this fall and he wants to get a new phone.  I mentioned to him that I thought he just got a new phone last summer.  (Taking another family member's renewal.)
I would like to confirm this.  Is there a way to look at the actual date that a phone was added to a given phone number in the plan?  (Or even a device history per phone line would be great.)  Due to renewal borrowing, the renewal date listed is not always accurate.

Answer 1

I am aware of the next upgrade date on the web site.  What clouds the issue in my case is that a line (or lines) has borrowed the upgrade of another line on the plan.  For example:
Line 1: Next upgrade is 1-1-15
Line 2: Next upgrade is 5-1-13
On 6-1-13 the user of line1 breaks his phone.  We see there is an available upgrade for line 2.  We use (or borrow) the upgrade for line 2 to get a new phone for line 1.  The new upgrade dates are now:
Line 1: 1-1-15
Line 2: 5-1-15
When I look at these dates it does not tell me the date a new device was added to a given line.  I can't just subtract two years and get that answer (since I borrowed from one line to give to another.)
I was hoping to find a way online to determine when a device was changed for a given phone line.  The closest thing available is what rcschnoor mentioned, the online receipts.