Riddles and Puzzles

Riddles and Puzzles # 73

My first is a number, my second another,
And each, I assure you, will rhyme with the other.

My first you will find is one-fifth of my second,
And truly my whole a long period reckoned.

Yet my first and my second(nay, think not I cozen),
When added together will make but two dozen.

How many am I?