When adding a subscription to a member why do the start date and subscribed until date default to the same value?

A member's subscription record records their subscription in perpetuity (or forever) unless it is stopped. It does not represent a single period of membership. Each time a member renews their subscription is extended by the period of the subscription.

The two dates on the subscription record show when that period of subscription began, the start date - this becomes an historical marker in time when that period of subscription began and the date that the subscription is subscribed up until, the subscribed until date - this is the date that is advanced by the period of the subscription each time an invoice for that subscription is created.

When a new subscription is added to a member both the start date and the subscribed until date default to today's date. This presumes that the period of subscription begins today and that the member has not yet had their initial period of subscription invoiced. When the initial period of subscription is invoice the subscribed until date will be advanced by the period of the subscription. For scenarios where the member's subscription is due on the anniversary of their joining this is all that needs to be done. For scenarios where the member's subscription is due on a fixed date and MemNet has been configured to calculate a pro rata amount for the initial subscription period, MemNet will determine the pro rata amount and nothing else needs to be done.

It is possible to modify both the start date and the subscribed until date when adding or editing a member's subscription. You may wish to edit the start date because the member's subscription began a few days ago but is only being added to MemNet now, or you may have rules that dictate that all new memberships begin on the first day of the next month or something similar. You may want to modify the subscribed until date so that all member's subscriptions are due at the same time but you do not use pro rata, or you may offer an initial period of free membership so their initial payment is not due to some time in the future, or the member may have already paid the initial period of membership when joining so is not due until the beginning of the next period. All of these modifications will produce a warning as they are not as the system expects, but this does not stop you from making these modifications.