Random Riddle: 5-10-2016

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?
 

Riddle