my broker uses the closing data as both the buy and sell date and that's how it reports those trades to the IRS. true dates don't matter because a short sale that is not hedged is always a short-term capital gain. there is an exception if those options are regulated futures contracts under code sec 1256, then they get reported - no detail needed - on form 6781 and split 60% long-term 40% short-term regardless of the actual holding period.
