In the sentence
A lasted till the bottom of B
where B is an event which, in its turn, lasted some years, "bottom" is the beginning of B or the end of B?
I found it in an economic article, the actual sentence is
the regulatory wave lasted till the bottom of the world 2007-2009 crisis
(Henry Penikas, "History of Banking Regulation as Developed by the Basel Committee on Banking Supervision in 1974-2014 (Brief Overview)", 2015).