"Not a second more, not a second less."
Translation:Ni un segundo más, ni un segundo menos.
1) It wouldn't be right in Spanish, because in that case "no un segundo más, no un segundo menos" would literally mean "don't a second more, don't a second less"; which makes no sense.
2) But it would be right in English: That's why we use "no/not" to express an exact amount as in: "ni más ni menos que...", meaning: "no more, no less than..."
In this case, the whole sentence would be: "Me tardé ni más ni menos que (sólo) cinco segundos. Ni un segundo más, ni un segundo menos", mening: "It took me no more, no less than (just) five seconds. Not a second more, not a second less."
I was going to use Ni un segundo, but checking the hover hint (what had only "no") I used No the "ni and the following negative. These hover hints are counter productive to the learning process. I had to "unlearn" what I thought was correct then "relearn" it again. Maybe this a a basswkards cognitive conditioning.
Because the speaker is traying to say it took them an exact amount of time to do something: It didn't took them ×+1 or x-1 seconds. No, it took them exactly × seconds.
Please, read my other replies on this thread to catch the whole meaning behind the idea of "ni un segundo más, ni un segundo menos".
This is a common frase in Spanish. Also, you may come across this "grammatical formula" when having two options and choosing neither of them (which, of course, meant you had more than those two options).