Related Clues:

• Improves, as whiskey
• Nears retirement
• 1 billion years
• Important periods
• 365 billion days, astronomically
• Interminable stretch
• They may be checked at the door
• Almost eternity
• Longish span of time
• Millennia on end
• Bronze and Iron, e.g.
• Ever so long
• Geological span
• Becomes drinkable, perhaps
• Grows old gracefully