"I am not in Moscow now."
Translation:Я сейчас не в Москве.
I'm not sure what "part of the message" is supposed to mean either. However, because of the intricate case system in Russian, word order is much more flexible than in English. Having said that, words toward the front of the sentence tend to have more emphasis than the words toward the end. And new information tends to be at the front of the sentence. Adverbs of time are often emphasized in conversations, so Russian likes to have them in the front and not at the end. (Where are you? Right now I am in Moscow.)
New information is at the end of sentences generally unless you use intonation to stress something somewhere earlier in the sentence. For instance Когда вы будете в Моске? Я там/в Москве сейчас. (When will you be in Moscow? I am there/in Moscow NOW). But...Где ты? Я сейчас в Москве.
Hi! Не сейчас negates сейчас, not в Москве. It's the equivalent in English of "I am in Moscow not now." It doesn't really work without a modifier: "I am/will be in Moscow not now, but tomorrow." Even then it doesn't really work. Я сейчас не в Москве negates the implied "am" and "In Moscow."
Technically speaking, it is in the locative case. However, in the modern Russian language, for the vast majority of nouns the endings are the same in the locative and in the prepositional cases. They only differ for roughly 150 nouns. (One example would be "лес": locative — "в лесу", prepositional — "о лесе".)
For that reason, the locative case is not even taught in Russian schools. So to your average Russian high school student "в Москве" would be in the prepositional case.
сейча́с (sejčás) [sʲɪˈt͡ɕas, ɕːas] "now; at once; just now": Univerbation of сей (sej, "this; this here") + час (čas), ”this hour”, ”this time” (note: “сей” is dated, bookish or stilted (unlike “э́тот”), “час” in the sense of “time” is dated or poetic). Hour and clock are both said ceas in Romanian, of the same origin as час.