When you already have a personalized OV-chipcard, any NS Flex subscription can start on the same day. You may however need to wait for the confirmation that your product can be "picked up" at a ticket machine, but you should get that the same day.

In case you don't have a chipcard yet and are ordering it together with a subscription, you'll only be able to start the subscription in 7 days. This is due to the card which has to be produced and sent to you by mail.

Do you mean you already have Flex and want to change the invoice date? There is only one way to do that: completely cancel Flex and remove it from you OV-chipcard (and confirm this at a ticket machine) and order it again on the day you want to receive the invoice.

In addition to Daniel: If you already have a season ticket, you cannot change the date anymore on which it monthly prolongs and get the invoice.

What you can do is order the same subscription on a new (usually free) ov-chipcard and cancel your current subscription when the new subscription becomes actieve (7 days after ordering).