Yesterday night after tzeis hacohavim, I accepted on my self a fast from alos haschar until tzeis hachoavim for today. I was reading the shulchan aruch and he says in siman 562 that it is not considered a taanins unless it was acceted before the day. Did I fulfil my neder and do I say annenu during mincha today?
You are very right, a kabalah to fast after it is no longer daytime doesn’t mean anything and you don’t say anaynu. Regarding making a neder, I have to know more about it. Did you make the neder to fat specifically today or to fast one day in general?