When first going to the Gmail restore tab within your Spanning backup account, a 90-day old date is automatically inserted into the 'From' date range filter at the top. This date is inserted into the 'From' filter field so that all of your most recent emails are loaded instantly, and you don't need to wait for all of your backed up data to load.
If you want to restore data outside of this 90-day window, you can either edit the date in the 'From' date range filter, or remove it completely to see all of your backed up data.
Now, if you select all of your mail to be restored, you can be certain that all email that Spanning has ever backed up will be included in the restore.