The ACS Survey provides the most up-to-date census estimates for US population groups. The 5 year survey is the most comprehensive. For example, the most recently available 5 year survey as of Sept 2015 is the 2009-2013 ACS 5 Year Survey.
I'm trying to calculate population changes since that survey was taken with a number of methodologies, but what I don't know is when the effective estimate date is supposed to be. For the 2009-2013 survey, is it January 1, 2013? Is it December 31, 2013? Is it some date in 2012? I'm assuming it is January 1, 2013, but have no way of knowing this other than an educated guess.