I have some short options expiring on December 31. I expect them to expire worthless so if I let them expire, will the premium affect this year's taxes or next year's taxes? They expire on the 31st, but I think that the gains won't settle until the following day which is next year.
Next year if the trade settles the following day. You will see in your transaction logs something indicating that fact as the contract will be removed from your portfolio. so if the log shows it the next day then your okay for 2016.
http://fairmark.com/investment-taxation/capital-gain/stocks-and-other-securities/last-day-sell/ Doesn't specifically mention options though.