The human race begins on Earth with Adam, possibly exactly 4000 years BC. God is therefore “saying” that He has started to count in thousands of years rather than some lesser quantity such as hundreds, tens or even ones. At the other end of the Earth experience we have Christ’s millennial (1000 year) reign (Rev.20:6). That accounts for 5000 years. Stuck in the middle are the AD years, 2009 of them.