Obavijestit ću vas čim saznam točan datum.

This is the normal way Croatian forms the future tense in this pattern.

The future is made with:

• the infinitive of the main verb
• plus the clitic form of htjeti: ću

When the infinitive comes before ću, the final -i of the infinitive is usually dropped:

• obavijestiti + ću → obavijestit ću

So:

• obavijestit ću vas = correct
• ja ću vas obavijestiti = also correct
• obavijestiti ću vas = not standard

Croatian often leaves out subject pronouns when the verb form already shows who the subject is.

Here, the speaker is understood from:

• ću = I will
• saznam = I find out / I learn

So ja is not necessary. You could add it for emphasis:

• Ja ću vas obavijestiti čim saznam točan datum.

That would sound like I will inform you..., with extra emphasis on I.

Vas is the object form of vi.

It can mean:

• you plural
• you polite singular

In this sentence it is used because obavijestiti takes a direct object:

• obavijestiti koga? → vas

So it means inform you.

Because after time words like čim Croatian normally uses the present tense, even when talking about the future.

So Croatian says, literally:

• I will inform you as soon as I find out the exact date

This is similar to English, which also says:

• as soon as I know not
• as soon as I will know

Also, saznati is a perfective verb, so its present-tense form often refers to a single completed future event in this kind of clause.

Because znati and saznati do not mean the same thing.

• znati = to know (a state)
• saznati = to find out / learn (the moment of getting the information)

This sentence is about the moment when the speaker obtains the exact date, so saznam is the natural choice.

Compare:

• Čim saznam točan datum... = As soon as I find out the exact date...
• Čim znam točan datum... = not natural here

Čim means as soon as.

It shows that one action happens immediately after another:

1. I find out the exact date.
2. I inform you.

So čim is more specific than just when. A word like kad/kada can mean when, but čim emphasizes immediacy.

It is in the accusative, because it is the direct object of saznam:

• saznati što? → točan datum

The noun datum is masculine singular and inanimate. For many masculine inanimate nouns, the accusative singular looks the same as the nominative singular.

So:

• nominative: točan datum
• accusative: točan datum

That is why there is no visible change here.

Yes. Croatian word order is quite flexible.

You can also say:

• Čim saznam točan datum, obavijestit ću vas.

This is equally natural. The difference is mainly in emphasis:

• Obavijestit ću vas čim saznam točan datum. = starts with the main action
• Čim saznam točan datum, obavijestit ću vas. = starts with the time condition

One thing to remember is that clitics like ću have special placement rules, which is why you get patterns like:

• obavijestit ću
• ja ću obavijestiti

Yes, obavijestiti is perfective.

That means it refers to a completed, one-time action: to inform someone. In this sentence, the speaker is talking about one future act of informing you once the date becomes known.

That fits the meaning very well:

• saznam = I find out
• obavijestit ću = I will inform

Both actions are seen as single completed events, not ongoing processes.