As impetus builds behind the BEPS Multilateral Convention, with more and more countries signing and ratifying, the question to consider is when will the new provisions apply?
There are two different issues: withholding taxes on the payment of interest, dividends and royalties and provisions affecting the taxable presence, overall profit or dispute resolution rights of companies and individuals.
The OECD secretariat has just published a note on the effective date for withholding tax – which is 1 January. The question put is which 1 January? 
Was it the one immediately following the expiration of the three months after the second country ratified – or was there a further year’s grace? The note sets out the legal advice from OECD Directorate for Legal Affairs, which states that it’s the first date – and there is no extra year. The problem arose because of slightly loose language in the English version of the Convention. Naturally the Vienna Convention on Treaties was considered.
A similar question arose in the UK, at least, on the effective date of the taxable period changes. Some argued that the changes took effect by reference to a company’s accounting period – not the country’s overall financial year. HMRC disagreed and clearly state that changes apply from 1 or 6 April, as appropriate.