Do you have a link for that tweet?
http://archive.is/o7HHw there are two tweets and they are probably still up at @HillaryClinton. The second one says something like “four minutes has passed that Donald Trump could have launched a nuclear weapon.”
Very irresponsible imho.
Snopes cited it while trying (and failing) to debunk it.
At 8:35 PM on 19 October 2016, Clinton published a tweet on the same subject...