The most common mistake I see in companies running Polish operations from abroad looks like this. An invoice is created on 3 March, the file sits in the ERP, and the transmission to KSeF happens on 6 March. Nobody is worried, because the 15 April deadline is weeks away. In fact that company just used offline24 mode without knowing it, and missed its deadline by two business days.
This is not a technicality for tax lawyers. Administrative penalties under Article 106ni of the Polish VAT Act start applying on 1 January 2027, and fiscal-criminal liability applies already. Below is a breakdown of both deadlines, mode by mode.
Two separate clocks
Two things get merged in everyday conversation, and they should not be.
The issuing deadline comes from Article 106i of the VAT Act and has nothing to do with KSeF. It applied before 2026 and still applies. The Ministry of Finance confirms this in its official table of invoicing modes, which states plainly that statutory invoicing deadlines remain unchanged.
The KSeF transmission deadline is the new layer. It depends on the mode you are operating in, and it is measured in business days rather than months.
You can comply with the first and breach the second. You can also breach both, if a March invoice reaches KSeF in May.
The issuing deadline: the 15th-day rule
The general rule under Article 106i(1) of the VAT Act: an invoice must be issued no later than the 15th day of the month following the month in which the goods were delivered or the service performed.
Paragraph 2 extends the same rule to advance invoices (faktura zaliczkowa). What counts there is the month you received the payment, not the month you deliver.
Example. You complete a project on 27 March 2026. The invoice must be issued by 15 April 2026 at the latest. If you also received a 30 percent advance on 10 March, the advance invoice was due by 15 April as well, counted from the payment month.
Special deadlines
Article 106i(3) and (4) carve out several exceptions:
Situation Issuing deadline Construction and construction-assembly services 30th day from service completion Delivery of printed books 60th day from release of the goods Book printing services 90th day from performing the activity Invoice on request from a private individual, requested within the transaction month 15th day of the following month Same, but requested later 15 days from the date of the request
If you operate in construction, the 30-day clock is your real deadline. The 15th-day rule is not.
No earlier than 60 days in advance
Article 106i(7) closes the window from the other side: an invoice may not be issued earlier than the 60th day before delivery, service completion, or receipt of payment.
Paragraph 8 provides an exception for continuous supplies and services settled in billing periods, provided the invoice states which billing period it covers. Subscriptions, leases, and maintenance retainers fall here.
For foreign-owned entities this matters more than it looks. Group billing calendars that issue quarterly invoices at the start of a quarter can collide with the 60-day limit on the Polish entity.
The issue date under KSeF: P_1 or the transmission date
This is the part that surprises people most.
Under Article 106na(1) of the VAT Act, a structured invoice is deemed issued on the day it is transmitted to KSeF. But the Ministry of Finance clarifies that this rule applies where the invoice reached KSeF on the same date entered in field P_1 of the FA(3) logical structure.
When the two dates diverge, Article 106nda(16) applies and the issue date becomes the P_1 date. That sounds like relief, but it works the other way. According to the Ministry's guidance published with the offline24 description, such an invoice is treated as issued in offline24 mode, with all the obligations that mode carries.
So a late transmission is not a "delayed online invoice". It is an offline24 invoice whose sending deadline has already expired, issued without the required QR markings.
How long you have, mode by mode
Mode When it applies KSeF transmission deadline Online Default, system operating normally Same day, transmission date should match P_1 Offline24 Your own decision: no connectivity, software failure, slow processing Immediately, no later than the next business day after issuance Offline, system unavailability The Ministry announced planned or sudden unavailability 1 business day from the end of the unavailability Emergency mode (tryb awaryjny) The Ministry announced a failure in its Public Information Bulletin 7 business days from the end of the failure Total failure Announced through mass media The invoice is not sent to KSeF at all
Source: table of invoicing modes, ksef.podatki.gov.pl.
Two details that are easy to miss.
First, the "24" in offline24 does not mean 24 clock hours. It means the next business day. An invoice issued on Friday evening is due by the end of Monday, unless Monday is a public holiday. Poland has more of those than most foreign finance teams expect, including several that move each year.
Second, if the Ministry declares a failure after you issued the invoice but before you sent it, your deadline automatically extends to 7 business days from the end of that failure. This follows directly from the Ministry's offline24 page.
Offline24 is not just a different deadline
Issuing outside the system triggers extra obligations. An invoice made available to the buyer outside KSeF before transmission must carry two QR codes: one marked OFFLINE, one marked CERTYFIKAT. Generating the second one requires a KSeF certificate of type 2.
You obtain that certificate in advance, not during an outage. This is exactly the item companies remember at the worst possible moment.
Corrections have their own trap. If you are correcting an invoice issued in offline24 mode, you must first transmit the original to KSeF and obtain its KSeF number, and only then issue the correction. The order is fixed.
What happens if you are late
Two separate regimes apply.
Fiscal-criminal liability for issuing an invoice late or defectively comes from the Fiscal Penal Code and operates independently of the KSeF rollout calendar.
Administrative penalties under Article 106ni of the VAT Act, covering among other things failure to transmit an invoice to KSeF on time, apply from 1 January 2027. That is not an invitation to relax through 2026. It is a window to fix the process.
There is also a plainer consequence than penalties. The VAT tax point arises regardless of whether you issued the invoice on time. Being late does not move your settlement, it just creates corrections and awkward conversations with your Polish accountant.
Checklist: how to stop missing the sending deadline
Make KSeF transmission automatic and immediate on approval, not a separate task someone does later.
Verify that the P_1 date matches the actual transmission date. A mismatch silently moves you into offline24.
Obtain a type 2 KSeF certificate before you need it.
Keep a visible queue of invoices awaiting KSeF confirmation, with the sending deadline shown per document.
Monitor Ministry announcements about unavailability and failures. They determine which deadline binds you.
If you are in construction or publishing, encode your special deadlines into the process instead of relying on the 15th-day rule.
After any outage, verify that every queued document actually went out. Silence in a system is not confirmation.
Summary
The deadline for issuing an invoice and the deadline for sending it to KSeF are different obligations, measured in different units, from different legal bases. The first gives you weeks. The second usually gives you one business day.
Biurko starts the KSeF transmission as soon as the invoice is approved, and automatically retries offline transmissions once an announced outage ends. Anything that misses its window is moved to a separate review queue with a notification, rather than disappearing into logs.
Create a free account at biurko.io and see how deadline handling works without manual tracking. The free plan is permanent, not a trial period.
FAQ
What is the deadline for issuing an invoice in Poland? Under Article 106i(1) of the VAT Act, no later than the 15th day of the month following the month of delivery or service completion. The same date applies to advance invoices, counted from the month the payment was received. The KSeF rollout did not change this deadline.
How long do I have to send an invoice to KSeF? In online mode the invoice should reach the system on the date entered in field P_1. In offline24 mode you have until the end of the next business day after issuance. If the Ministry of Finance declares a system failure, the deadline is 7 business days from the end of that failure.
What happens if I send an invoice to KSeF a day late? According to Ministry of Finance guidance, the invoice is then treated as issued in offline24 mode, with the P_1 date as its issue date. That means you were expected to meet the requirements of that mode, including two QR codes, and to transmit by the next business day.
Can I issue an invoice in advance? Yes, but no earlier than the 60th day before delivery, service completion, or receipt of payment (Article 106i(7) of the VAT Act). The exception covers supplies and services settled in billing periods, provided the invoice states which period it covers.
When do penalties for late KSeF transmission start? Administrative penalties under Article 106ni of the VAT Act apply from 1 January 2027. Independently of that date, fiscal-criminal liability for late or defective invoicing under the Fiscal Penal Code applies throughout.