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!
**Disclaimer: Effort has been made to offer correct information; but due to the discussion forum limitations, the poster disclaims any legal responsibility for the accuracy of the poster's response**
