Learn Hardcore Japanese: kaigi no zikan ha kyou kimarimasu. - The meeting time will be decided today.

It links two nouns and makes an “of” relationship. 会議の時間 literally means “the time of the meeting.” の is the default way to connect two nouns in this attributive/possessive way.

Why is は used after 時間? Could I use が instead?

は marks the topic (“As for the meeting time…”). It assumes the meeting time is already known or introduced and sets it as the topic for what follows.
が marks the grammatical subject and tends to present it as new or focused information. Both can be possible:
会議の時間は今日決まります。 Neutral/topic: “Regarding the meeting time, it will be decided today.”
会議の時間が今日決まります。 Focused: “It’s the meeting time that will be decided today.” (for instance, as opposed to the place)

Why isn’t there an を object marker? Isn’t “decide” a transitive verb?

Japanese has a pair:

決める (transitive) = “to decide (something)”
決まる (intransitive) = “(something) is decided / will be decided” Your sentence uses the intransitive verb 決まる, so the thing that becomes decided (会議の時間) is the subject/topic, not a direct object. If you use the transitive verb, you’d say:
（私たちは）会議の時間を今日決めます。 “We will decide the meeting time today.”

Does 決まります mean “will be decided” even though it’s the present tense?

Yes. Japanese “non-past” (～ます／～る) covers both present and future. With a time adverb like 今日, it naturally means the future: “will be decided today.”

Can I say 今日に決まります? What’s the difference?

Be careful:

会議の時間は今日決まります。 means “The meeting time will be decided today.” Here 今日 is when the decision happens; no particle needed.
XはYに決まります。 uses に to mark the chosen result/value. For example: 会議の時間は3時に決まりました。 “The meeting time was set to 3:00.” So 今日に決まります would mean “(something) will be set to today.” That makes sense with a date (e.g., 会議の日は今日に決まりました “The meeting date was set to today”), but not with 時間 (time-of-day) because “today” is a day, not a time.

Where can 今日 go in the sentence? Is the word order flexible?

Time expressions are flexible. All of these are natural, with slight emphasis differences:

今日、会議の時間は決まります。
会議の時間は今日決まります。
会議の時間は、今日、決まります。 They all mean the same thing in most contexts: the meeting time will be decided today.

Does 時間 here mean “time of day” or “duration”?

By itself 会議の時間 can mean either, depending on context. If you specifically mean the start time/clock time, clearer options are:

会議の開始時間 (start time of the meeting)
会議の時刻 (the specific clock time) Example: 会議の開始時間は今日決まります。

How do I say “We will decide the meeting time today” (explicit “we”)?

Use the transitive verb 決める:

（私たちは／担当者は）会議の時間を今日決めます。 In real Japanese, the subject (we) is often omitted if clear from context.

What’s the difference between 決まる, 決める, and 決定する?

決まる (intransitive): “be decided” (natural, everyday)
決める (transitive): “decide (something)” (everyday)
決定する (formal): “determine/decide” (often used in official/business contexts). Transitive: 予定を決定する; Passive: 予定が決定される (“the schedule is determined”). Your sentence uses the intransitive everyday form. A formal business version might be:
会議の開始時間は本日中に決定されます。 or …決定いたします。

What’s the nuance difference among 決まります, 決まりました, and 決まっています?

決まります: non-past; here “will be decided (today).”
決まりました: past; “has been decided/was decided.” Often used to announce completion of a decision.
決まっています: state resulting from a past decision; “is already decided.” Example: 会議の時間はもう決まっています。

How would I ask “When will the meeting time be decided?”

会議の時間はいつ決まりますか。 If you want “Who will decide (it)?”, switch to the transitive verb:
誰が会議の時間を決めますか。

Are spaces normal in Japanese like in the given sentence?

No. Japanese usually doesn’t use spaces between words. The spaced version is a learner aid. The normal writing is:

会議の時間は今日決まります。

How do you pronounce the sentence?

Kana: かいぎのじかんはきょうきまります。
Romaji: Kaigi no jikan wa kyō kimarimasu. Key readings: 会議(かいぎ), 時間(じかん), 今日(きょう), 決まります(きまります).

Is using the passive 決められます natural here?

Generally, to say “will be decided,” Japanese prefers the intransitive 決まります. The passive 決められます is often understood as the potential of 決める (“can decide”) and can be ambiguous. For a formal passive, use:

会議の時間は今日決定されます。 But for everyday neutrality, stick with:
会議の時間は今日決まります。