By popular request we've added a feature that will allow you to create sessions in the past. Any Check-Ins Editor will now see a "Add past sessions" button on the settings page of an event.
From there you'll be able to add one specific session, or add multiple sessions to any event. While adding one specific session will allow you to insert a date anywhere, multiple sessions will only allow you to create a set of sessions before the very first created session.
Do you have data from a time before you used Check-Ins? Well now you can add all of that information in to keep it all in one place.
The Check-Ins Team