Yes.  An invoice's service start date can be prior to the invoice date.   The same applies for bills.

For invoices and revenue recognition, this can happen if services are delivered prior to a customer invoice being raised.  

Flowrev will defer on the invoice date (when invoice lines are mapped to a revenue account), and recognize from the service start date (onwards).

There are no limitations on the duration you can set, and your service start date and end date on bills and invoices can be as far apart (or not) as is necessary. However, your subscription may support schedules only of a certain duration.

Your country's tax authority may have limitations on the duration within which ratable revenue needs to be recognized.  In this case, you can use the Balloon recognition method for those lines should you want to take a conservative revenue management stance, and recognize revenue in accordance with tax regulations.  Alternately, you can work on a financial basis, recognizing revenue over the full duration of service, and make adjustments for taxes separately.