How do they figure out the star date in Star Trek series ?

Its weird they don't use dates like we do like Dec. 4th 2010 they use star dates like Star Date 45652.1.( this is taken from a TNG episode). Now I know TNG takes place in the 24th Century so why isn't it a date some where between the years of 2301–2400. how do you figure out what the date is by using the stardate?