Currently, When we create back dated Subscription then invoice date of generated Invoice is always a subscription activation date. For Ex . If today is 22nd may and I am creating back dated subscription starting from 15th may, then Invoice date of generated Invoice will be 15th may.
So, In Indian Market, This is general scenario, where Owner may allow his customer to use his product (by subscription) and product owner may want to create subscription and send invoice to customer after some days. So in this case, invoice date should be the date when owner is creating subscription. There should be flexibility to choose invoice date for subscription if subscription is being created on today.