Is it permissible to say "time is treacherous," knowing that time is part of [the concept of] الدهر (ad-dahr, i.e., fate/time), and it was forbidden to revile ad-dahr in the Hadith Qudsi: "Do not revile ad-dahr, for Allah is ad-dahr"?

It is not permissible to say "time is treacherous" because time has no power to act; rather, it is Allah who manages the universe and directs its affairs. The Prophet, peace and blessings be upon him, forbade cursing time because it amounts to cursing Allah. Sheikh Ibn 'Uthaymeen clarified that expressions involving cursing time are forbidden, such as "time is treacherous," because treachery is a blameworthy characteristic. However, if such a phrase is uttered merely as a statement of fact, there is no harm in it, such as saying, "This is a difficult day." If one says, "Oh, the misery of the day I saw you," intending their own misery, there is no harm. But if the intention is to curse time, then it is not permissible.