Okres rozliczeniowy kończy się dzisiaj.

Because kończyć się is a very common Polish pattern meaning to end / to come to an end.

• kończyć coś = to end something / to finish something
  • Firma kończy projekt. = The company is finishing the project.
• kończyć się = to end
  • Projekt kończy się jutro. = The project ends tomorrow.

So in your sentence:

• Okres rozliczeniowy kończy się dzisiaj.
• literally: The settlement/billing period is ending itself today
• natural English: The billing period ends today.

The się is not optional here if you want this intransitive meaning.

Here się does not mean himself/herself/itself in a literal English sense. In this sentence, it is part of the verb expression kończyć się.

It helps turn the verb into something like:

• to end
• to come to an end
• to be ending

So:

• kończyć = to finish something
• kończyć się = to finish/end

Many Polish verbs work this way, where się changes the meaning or makes the verb intransitive.

Kończy is the 3rd person singular present tense form of kończyć.

Conjugation in the present tense:

• ja kończę = I finish / am finishing
• ty kończysz = you finish
• on/ona/ono kończy = he/she/it finishes
• my kończymy = we finish
• wy kończycie = you all finish
• oni/one kończą = they finish

Since okres rozliczeniowy is singular, the verb must also be singular:

• okres ... kończy się

Because Polish usually does not need subject pronouns when the subject is already clear.

In English, you must say:

• It ends today.

In Polish, the subject is often just the noun itself, or even omitted entirely if the context makes it obvious.

Here the subject is explicitly stated:

• Okres rozliczeniowy = the billing/settlement period

So there is no need for any extra pronoun like it.

It is in the nominative case, because it is the subject of the sentence.

Breakdown:

• okres = noun, masculine singular nominative
• rozliczeniowy = adjective, masculine singular nominative, agreeing with okres

Polish adjectives must match the noun in:

• gender
• number
• case

So rozliczeniowy is shaped the way it is because it describes okres.

Rozliczeniowy is an adjective meaning something like:

• billing
• accounting
• settlement-related

It comes from the noun/verb family related to rozliczenie / rozliczać:

• rozliczenie = settlement, accounting, billing, reconciliation
• rozliczać = to settle accounts, bill, account for

So:

• okres rozliczeniowy = billing period, settlement period, or accounting period, depending on context

This is very common in formal, business, tax, HR, and utility contexts.

Yes, Polish word order is flexible. Dzisiaj can move, although the most neutral version here is:

• Okres rozliczeniowy kończy się dzisiaj.

You could also say:

• Dzisiaj okres rozliczeniowy kończy się.
• Okres rozliczeniowy dzisiaj kończy się.

These are grammatically possible, but they may sound more marked, stylistic, or focused differently.

In general:

• end position often sounds natural for time expressions
• fronting dzisiaj gives it more emphasis: Today is when it ends

So the original sentence is a normal, neutral word order.

Yes. Dziś and dzisiaj both mean today.

So these are both correct:

• Okres rozliczeniowy kończy się dzisiaj.
• Okres rozliczeniowy kończy się dziś.

Difference:

• dzisiaj is a bit fuller and slightly more neutral in many contexts
• dziś is shorter and very common in both speech and writing

In everyday Polish, both are natural.

Because Polish, like English, often uses the present tense for scheduled or currently unfolding situations.

So:

• kończy się dzisiaj literally looks like is ending today / ends today

This works well for:

• schedules
• deadlines
• periods of validity
• events with a known endpoint

It does not have to mean the ending is happening at this exact second. It can mean that today is the day on which the period ends.

This is mainly about aspect.

• kończyć się = imperfective
• skończyć się = perfective

In practice:

• kończy się dzisiaj = ends today / is ending today
• skończy się dzisiaj = will end today / will be over today

Important point: in Polish, a perfective verb does not normally have a true present-tense meaning. Its present-looking form usually refers to the future.

So:

• kończy się = present
• skończy się = future

The imperfective version often sounds more neutral in schedule-like statements. The perfective version can sound a bit more like focusing on the completed endpoint.

A rough pronunciation guide:

• Okres ≈ OH-kress
• rozliczeniowy ≈ roz-lee-cheh-NYOH-vih
• kończy ≈ KON-chih
• się ≈ shyeh or syenh (depending on how detailed you get with nasal vowels)
• dzisiaj ≈ JEE-shai

A few useful pronunciation notes:

• cz sounds like ch in chocolate
• ci before a vowel often sounds like a softened ch/ty-like sound
• dz is a voiced sound, like ds said together
• się contains the nasal vowel ę, though in normal speech its pronunciation depends on the following sound

Polish stress is usually on the second-to-last syllable:

• o-KRES
• roz-li-cze-NIO-wy
• KOŃ-czy
• DZI-siaj

Yes, it sounds fairly neutral-to-formal because of the phrase okres rozliczeniowy, which belongs to administrative, business, tax, payroll, or service-related language.

You are likely to see it in places like:

• invoices
• contracts
• subscription information
• payroll documents
• tax or accounting notices

The grammar itself is not especially formal, but the vocabulary is typical of official or business contexts.