You can always play it safe and file the 2017 1120-S. As I'm sure you're aware (and in case you just need a refresher) startup costs incurred prior to the business being open can be deducted in that first year up to a maximum of $5K. Then any remaining is Amortized (as opposed to capitalized) and deducted over the next 15 years. Now with the TurboTax program it's sometimes necessary to enter startup costs manually in the Assets section by selecting the asset class "other" and working it through manually. Usually though (but not always) when the program asks for startup costs it gets it right.
As I see it, the reason the IRS is vague on opening date, is because it's usually defined by the state. For example, here in FL where I live your S-Corp opening date is the date your business is "active" as indicated by the active date on the registration. I myself don't know anyone in this state who registers a business otherwise, myself.
In other states (and this is what I hear, not facts I've verified) the active business date must be identified in the articles of incorporation and can be after the registration date and doesn't have to be in the same tax year of registration.
But what I've seen most common on this forum regardless of state, is that folks have been told to consider the business active on the date of registration and file anyway. One of the reasons behind that is to avoid the possibility of finding out the hard and costly way that a return was supposed to be filed in the year of registration. That $125 per month per owner late filing fee can easily contribute quite a bit to bankrupting a new corporation before it even gets off the ground. I've also never heard of a corporation being fined or penalized for filing a return that was not needed.
@TaxGuyBill is much more versed on this S-Corp stuff than I am, and I'm sure if he has more to offer he'll jump in here.
