I'm getting a pretty decent tax return this year and I still need to fully contribute to my IRA. If I plug into my tax software that I contributed the full amount (5500), my return is greater and that would give me enough to fully invest for 2016. Is this doable, using tax return money to contribute to the previous year's investment? Seems technically doable and to me ethically fine, but I'm unsure of the legality.
As long as your broker receives your contribution before your tax deadline, you're fine.