If it the date of your Calling Plan, try to Change your Calling Plan. It may show dates of your contract at the top.
If it is dates of your BB, this is usually linked to your Calling Plan.
1999: Freeserve 48K Dial-Up => 2005: Wanadoo 1 Meg BB => 2007: Orange 2 Meg BB => 2008: Orange 8 Meg LLU => 2010: Orange 16 Meg LLU => 2011: Orange 20 Meg WBC