In relationship management you also want the information to be complete and include relationships that were before CiviCRM was used. This could typically be relationships older than 20 years. CiviCRM imposes a limit and I cannot find the setting to change it to our needs.
For CiviCRM 4.6+ go to Administer menu > Customize Data and Screens > Date Preferences. From there, change the Start Offset of SearchDate.
I found this answer in Member since date before 1995 and although it talks about Membership dates, it apparently applies to Relationship dates too.