You can use your 2012 capital losses to reduce your current year’s income taxes by applying such losses against your 2012 capital gains. You must however be careful of the superficial loss rules preventing you from claiming a capital loss on an identical asset that you reacquired 30 days before or after the sale date.
If capital gains were realized in the years 2009 to 2011 and net capital losses were incurred in 2012 then you can carry these losses back against previous years’ capital gains. You can carry the unused 2012 losses forward to future capital gains.
The last 2012 transaction date effective for publicly traded securities is December 24, 2012.