Chaos, the years are arbitrary but the closer the target date to today, the more conservative (bonds:stocks) the asset allocation. You should target the closest target fund to your retirement date if you are happy with the allocation Vanguard has arbitrarily set for that fund.
Regardless of your actual chosen retirement year, if you want to be a little more aggressive than Vanguard "thinks" you should be, then choose a target year farther out than your actual retirement, and vice versa for more conservative.
San Francisco Area
ESR'd March 2010. FIRE'd January 2011.
As if you didn't know..If the above message contains medical content, it's NOT intended as advice, and may not be accurate, applicable or sufficient. Don't rely on it for any purpose. Consult your own doctor for all medical advice.