New LLC and Startup Costs

Per Publication 535 from the IRS, on page 29:

A start-up cost is amortizable if it meets both of the following tests.
• It is a cost you could deduct if you paid or
incurred it to operate an existing active
trade or business (in the same field as the
one you entered into).
• It is a cost you pay or incur before the day
your active trade or business begins.

For further reading and information, here is a link to Pub 535

https://www.irs.gov/pub/irs-pdf/p535.pdf

If your business is through your website, I would say your startup costs are up to the day your website went public.  Similarly as to a retail or other business...when do you actually become a trade or business with the public?   Generally, when you open your doors to/for the public.  

I strongly suggest reading Pub 535 information, and it may have links to other information you want/need as well.

Good luck!

Are my startup costs determined by the date I filed and received my state & federal documents (2/25/19), or are they all costs incurred up to when my website went public?

Neither. Startup costs are those costs incurred before you were "officially open for business". That "Open" date is determined by you. It's perfectly possible for you to have been open for business on 1/1/2019 which has nothing what-so-ever to do with your document dates or website publishing date. But do check for dates in your state and fed documents. If those documents give an "effective date" then I would recommend you establish your "open" date on or after the effective date in any of those state or fed documents.

I know I can only deduct $5k this year and will amortize the balance over the next 15 years.

It's only worth it to you to claim the full $5K in that first year, if you actually have $5K of taxable business income after all other business deductions are taken. SO keep that in mind.