On your first account/IBAN, you can visit the Export section. The first date you can select should be the day you've opened your account. Or choose a specific IBAN you want to know the initial date for and do the same.
Maybe you've a message in your mailbox received at account creation as well.
The invoice list for my account seems a little off. That billing date is 1 month and 1 day later than the actual date I opened my account at, possibly due to a free month.
And you can always ask support.