Hvis jeg ser forskellige datoer i kalenderen, bliver jeg i tvivl om, hvilken der er rigtig.

Hvis introduces a condition: if.

Here, the sentence describes a general pattern:

• Hvis jeg ser forskellige datoer ... bliver jeg i tvivl ...

That can feel close to whenever in English, because it is not about one single event, but about what typically happens in that situation.

Could når be used? Sometimes når can mean when/whenever, but hvis is the more natural choice when the speaker is presenting the situation as a condition: if this happens, then I react like this.

They mark clause boundaries.

1. Hvis jeg ser forskellige datoer i kalenderen,
   This is a subordinate clause introduced by hvis, so it is separated from the main clause with a comma.

2. ... bliver jeg i tvivl om, hvilken der er rigtig.
   Here hvilken der er rigtig is an embedded question, and Danish normally separates that clause with a comma too.

So the commas are doing grammatical work, not just stylistic work.

This is because Danish main clauses follow the verb-second rule.

The first position in the sentence is taken by the whole Hvis clause:

• Hvis jeg ser forskellige datoer i kalenderen

Once that first slot is filled, the finite verb of the main clause must come next:

• bliver

Then comes the subject:

• jeg

So:

• Hvis jeg ser forskellige datoer i kalenderen, bliver jeg i tvivl ...

If there were no opening Hvis clause, you would say:

• Jeg bliver i tvivl om, hvilken der er rigtig.

Because forskellige is the adjective forskellig in its plural form.

• datoer = dates, plural
• adjectives in Danish usually take -e in the plural

So:

• forskellige datoer = different dates

Compare:

• en stor bog = a big book
• store bøger = big books

The same pattern is happening here.

Kalenderen means the calendar.

In Danish, the definite article is often added to the end of the noun:

• en kalender = a calendar
• kalenderen = the calendar

The preposition i means in, and that is the natural choice here because the dates are understood as being written inside the calendar.

So:

• i kalenderen = in the calendar

This contains the fixed expression at blive i tvivl om noget.

It literally works like:

• blive = become
• i tvivl = in doubt / unsure
• om = about / as to

So the structure means:

• I become unsure about ...
• I start doubting ...

This is a very common Danish expression.

Because bliver focuses on the change into that state.

• jeg er i tvivl = I am unsure
• jeg bliver i tvivl = I become unsure

In this sentence, the idea is:

• first I see conflicting dates
• then I become uncertain

So bliver is exactly right.

Because i tvivl normally takes om.

You can be uncertain:

• om noget = about something
• om, hvad der er rigtigt = about what is correct
• om, hvilken der er rigtig = about which one is correct

So om belongs to the expression i tvivl om.

Because the sentence is asking about one correct date out of several possible dates.

• hvilken = which one, singular
• hvilke = which ones, plural

Even though datoer earlier in the sentence is plural, the speaker is trying to identify the single correct one.

So:

• hvilken der er rigtig = which one is correct

If more than one were correct, you would expect something like:

• hvilke der er rigtige

Because this is an embedded question, not a direct question.

Direct question:

• Hvilken er rigtig?

Embedded question:

• ... hvilken der er rigtig

In Danish, embedded questions do not use normal question word order. They use clause word order instead.

Also, der appears here because hvilken is the subject of the embedded clause.

A useful comparison:

• Jeg ved ikke, hvem der kommer.
  Here hvem is the subject, so der appears.

But:

• Jeg ved ikke, hvem du kender.
  Here hvem is the object, so there is no der.

In your sentence, hvilken is the one that is correct, so it functions as the subject of er rigtig.

Rigtig is a very common everyday word meaning right or correct.

So:

• den rigtige dato = the correct date
• hvilken der er rigtig = which one is correct

You could sometimes also hear korrekt, but rigtig is more natural in ordinary speech in a sentence like this.

Because the sentence describes a general situation or repeated experience, not one specific past or future event.

Danish often uses the present tense for this kind of pattern:

• Hvis jeg ser ... bliver jeg ...

This is similar to English sentences like:

• If I see different dates, I get confused.

So the present tense here expresses a usual reaction.