The W-2G form has a field for "date" but not one for "time of day", so if the casino informs the IRS that they paid the money on 12/31/99 I don't see how the player can decide to include it as income in 2000 instead of 1999. I think when the player signed the W-2G with the date of 1999, they committed themselves to reporting it as 1999 income. Just my humble opinion.

Ron