今は時間がありません I do not have time now
In the above sentence, "I" is the subject and time is the object. Therefore after the kanji for an hour, an o particle should be used. Then why is ga particle used?
In the above sentence, "I" is the subject and time is the object.
This is actually wrong. In the sentence, the subject is 時間, and the verb is ありません. 今 in the sentence, is the "topic", and the topic is NOT NECESSARILY the subject or the object. It's the main topic we're discussing here. As for "I", it's not mentioned in the sentence.
As for now, time doesn't exists. (Don't have time now).
You see, I put "as for" in English, because that's the most accurate translation of the は particle I could think of. The topic particle は marks the topic of the sentence, meaning the rest of the sentence are related to it, but the topic may not be the subject or the object. It could be, though.
As for school, everyone studies. (School is topic, not object, not subject)
As for cats, cats are cute creatures. ("Cats" is topic, also happens to be subject)
(In a context where we discuss what to eat)
As for fish, (I) don't eat fish. (Fish is topic of sentence, also happens to be object).
So again for your sentence, 今 is topic of the sentence, and the subject is 時間, because who's doing the action of ありません? It's 時間 that's doing the ありません, thus making 時間 the subject. Yes you could also modify the sentences to say "時間はありません", in which case now the 時間 is both the topic and the subject.