Weird values in gitlab_subscriptions table

Follow-up to https://gitlab.slack.com/archives/CBFCUM0RX/p1545129919229800

As I see most of the Gitlab subscriptions staring(start_date) when the namespace was created(created_at) based on the gitlab_subscriptions table.

In the namespaces table, we can't see when actually changed the plan id to a paid plan, so probably with the initialization of the gitlab_subscriptions the namespace creation was set to the start date as well, right?

/cc @jeremy