The number 21 refers to the 21st century.

Gregory refers the manuscript even later, to the 10th century.

Their beginnings are without doubt to be referred, at least, to the fourth, and possibly to the third century.

"Historic" here refers to the early twentieth century, as Cody was not incorporated until 1901.

The origin of the work may be referred with great probability to the 12th century.

Early islamic philiosophy refers to the time period of the 8th to the 12th century.

The "21" in Agenda 21 refers to the 21st century.

(The 3 denotes the third millennium; the 21 refers to the 21st century.)

I am referring to the sixteenth century when the discovery of America turned everything upside down.

It possesses poems the origin of which is referred with probability to the sixth century.