You are permitted to reimburse yourself for qualified medical expenses incurred anytime after the establishment of your HSA, so with regard to covering this expense with an HSA distribution, it doesn't matter when the contribution for the year is made. Because HSA contributions through your employer are not subject to Social Security and Medicare taxes, it's generally better to make all of your HSA contributions through payroll deductions. However, keep in mind that reducing the amount of income subject to Social Security taxes can potentially reduce the amount of your eventual Social Security benefits slightly because these benefits are calculated using your 35 highest earning years.
