"A decade is not a second."
Translation:Dekada to nie sekunda.
Ah, true. I think what confused me about your original comment was 'Here "to" is not necessary but it's not an error to put it.' - really that's not the case, it's "jest" that's optional in this case. Basically if you use "to", you don't have to worry about declining even if you also use być, but if you don't have a "to", you need to use the full być + instr. construction.
"normal sentences" :
dakada to nie sekunda
dekada to nie jest sekunda
dekada nie jest sekundą
rare: dekada nie jest to sekunda.
dekada to nie jest sekundą
Here "to" is used in a different meaning- it is an expressive particle. In this sentence I cannot think of context where it fits. but "A maj to nie jest miesiącem"?" ("And May is not a month?") or "A kobieta to nie jest człowiekiem?" (and a woman is not a human?)