Thus is his cheek the map of days outworn,
When beauty lived and died as flowers do now,
Before these bastard signs of fair were born,
Or durst inhabit on a living brow;
Shakespeare begins by continuing some of the themes in the previous two sonnets.  This sonnet refers to “days outworn" — times that have passed.  The Bard states that the Fair Youth’s face reflects — “the map of” — how he has lived. They are past days that have gone forever. 
 The most notable feature of this opening line is its conciseness; the characteristic economy of Shakespeare’s writing.
Sonnet 68