She says that everything has changed for me now, and I will be disappointed later if I do not keep a written record. Failing that, she assured me that she would be disappointed. In all honesty, I so prefer to guard my privacy that I do not think I would accept such a directive from anyone else, but my fondness for her is such that it seems only proper to follow her advice. In any case, there is no question but that today was nothing if not eventful. One moment, my life was mundane; mere hours later, the whole world seemed new and different.

ТС, "nothing if not" = чрезвычайно, крайне, в высшей степени.

В любом случае, не о чем говорить, кроме того, что сегодняшний день уж точно не отмечен никакими событиями.

наверное лучше всего написать автору и попросить перефразировать как-нибудь попроще!

He-Cat+
nothing if not - это не только at the very least, в том же Коллинзе указано и другое значение - "certainly", которое опять-таки использовано для усиления, типо "above all else".
Shakespeare used this idiom in Othello:
"IAGO
O gentle lady, do not put me to ’t,
For I am nothing, if not critical."

Т.е. "сегодня было очень очень eventful"

http://www.collinsdictionary.com/dictionary/english/nothing-if-not