Hi Eugene,
Unfortunately, our system does not have the capacity to store date ranges, e.g. "1980s" or "Between 1981 and 1989."
The best you can do is choose one date, use the "before this date" or "after this date" status indicator, and explain the issue in the text of the page.
From http://www.wikitree.com/wiki/Date_Fields#Date_Ranges