To manually set the Initial Subscription date, you need to edit the user's profile.

You do this via http://www.openvpms.org/users/xxxx where xxxx is the user's login name.  However, you may not know this.

The easiest way is to use the Who Bought What report to display the subscription purchases, and then click on the user's name. Note that if you click on the Email column heading, it will sort by email address and hopefully, all the subscription purchases by the one practive will be grouped together.

Clicking on the user's name will bring up the user screen as follows:

Click the Edit tab and then the Subscription Information sub-tab to bring up the following:

If the Initial Subscription date has never been set it will show today's date.  Set the date as appropriate and then click the Save button.