Why not "Teraz to dobry czas"?
"Teraz" undergoes Part Five in here.
OK so I can only use ''to'' as in "[noun] to [noun]"?
[Noun phrase] to [noun phrase]. Apart from that, yeah (at least so it seems to my sleepy mind right now).
Myślę, że lepiej jest "Now IT is a good time"
В добрый час, друзья, в добрый час!
Is there no declension here? Like 'Teraz jest dobrym czasem'.
No. In an "X is Y" sentence, Y can only take Instrumental if both X and Y are noun phrases (or if X is a personal pronoun and Y is a noun phrase).
"Teraz jest dobrym czasem" is as if you defined the word "now" by saying that it means "a good time".