Can I do Tax-loss harvesting selling on Dec. 31?

It says:   Timing: To count for the current tax year, all sales must be settled by the last trading day of the calendar year (usually December 31st).

But if I do it in my margin accts, it will settle on the same day.

Reporting: You must report your capital gains and losses on Form 8949 and Schedule D (Form 1040) when filing your taxes.