..."US Tax losses must clear before the end of the year."
That is not correct -- in the U.S. the trade only needs to take place before the end of the year. The settlement date is irrelevant to the IRS.
This is a direct quotation from IRS publication 550 (Investment income and expenses):
..."Do not confuse the trade date with the settlement date, which is the date by which the stock must be delivered and payment must be made.
..."You are a cash method, calendar year taxpayer. You sold stock at a gain on December 28, 2012. According to the rules of the stock exchange, the sale was closed by delivery of the stock 3 trading days after the sale, on January 3, 2013. You received payment of the sale price on that same day. Report your gain on your 2012 return, even though you received the payment in 2013. The gain is long term or short term depending on whether you held the stock more than 1 year. Your holding period ended on December 28. If you had sold the stock at a loss, you would also report it on your 2012 return."