I'll fully admit, I have zero experience in dealing with tax auction transactions with my client base, but this is an excellent question, so... @Michael Plaks challenge accepted!
On the date of the auction, you hold a the rights to the delinquent taxes and associated payments - all of the benefits of ownership that you have at that point are only related to those delinquent taxes, and the future rights you might obtain after 12 months. The general IRS test on determining acquisition date (this I have had to dig into a number of times) - do you have all the benefits and burdens of ownership, regardless of when title is transferred. I didn't dig too deep here, but at least one SC county notes on their website that your interest in the property doesn't actually vest until after the expiration of the redemption period. Following that up with when you have the benefit of ownership of real property - it isn't until you actually have taken possession of the property.
So then, the question comes, when that redemption period closes at the end of 12 months, does your holding period start with when you acquired the right to collect the delinquent taxes and interest? Or can you convey that original acquisition of this right to collect the delinquent taxes, which you then effectively exchange for the Deed if the redemption period has not been executed - but kept the same holding period?
My initial reaction from my walk through above would be that the holding period in the Real Estate doesn't begin until you have the rights and benefits associated with the real estate - so the earliest at the end of the redemption period, but really beginning once you take possession and can start doing stuff with the property.
Some further digging I found some examples of it, but if you were a paying client I would be spending more time to see if there were any subsequent changes to this rule - at the very least Bloomberg isn't calling it out on me ;) . Check out Rev. Rul. 72-200, 1972-1 C.B. 233 (If you can find it on Google, I can only find it in Bloomberg / Answer Connect) - in which case there was an IRS Tax Lien Sale, with a mere 120 day redemption period. It was determined that the holding period for the acquired real estate started after the end of the redemption period, given that is when the acquiring party started to enjoy the benefits of the real estate ownership. Assuming this revenue ruling still stands, it mirrors the SC situation nicely, meaning it would support your acquisition date of the real property not starting until you have taken possession.
I think this would be highly dependent on the terms of the local jurisdiction as well, so what might be correct in one jurisdiction, could be completely wrong in another depending on the rights acquired related to the property.
But happy to be pointed where I am wrong! In the end unless you are in an odd spot of selling the property within 12 months of the wrong date, it won't really matter for tax purposes.