How many am I?

It's pretty easy but I like the flow of this one. I'm new so sorry if you've all seen it before;

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?

Edit 1: Just wondering whether lack of response means no one knows, no one cares or everyone knows?