I cannot imagine how it would benefit society to provide full deduction of gambling losses, and can conceive of avenues for corruption to expand if it were offered.
What I stated above is true across fiscal years. If you win a $10,000 jackpot on December 31, and lose all $10,000 on January 1 (or at any point over the next 12 months), the above comment is true. You don't get to reduce your income in the next year by the $10,000 of losses, or edit the previous year's return, or offset the earlier win in any way.