Possible Clues:

• The 21st, e.g.: Abbr.
• Millennium div.
• 100-yr. span
• 1776-1876: Abbr.
• 20th, e.g.: Abbr.
• Div. of a millennium
• Period of 100 yrs.
• Five score yrs.
• Many yrs.
• 1200 mos.