In Poland, the official system for representing dates and times follows the international ISO 8601 standard.
In Poland, the first system for denoting abbreviated dates used Roman numerals for months (e.g., 11 XI 1918 for Independence Day).
The authorities changed the order of the date stamps in 1979 to follow Polish industrial standard PN-90/N-01204 (Polskie Normy) similar to ISO 8601; 1981-12-13 has been the preferred format since then.
A 12-hour clock is commonly used in speech when unambiguous (usually amongst people who live in or come from the Anglosphere), with the AM/PM distinction denoted by phrases rano ("in the morning"), po południu ("in the afternoon"), wieczorem ("in the evening"), w nocy ("at night"), and nad ranem ("before daybreak" or "in the wee hours") when needed; written communication uses 24-hour clock almost universally, including written forms of informal speech.
Times that introduce ambiguity in 12-hour notation (12:00AM and 12:00PM) are sometimes avoided in speech and replaced by "noon" and "midnight", but in the absence of further clarification, 24-hour interpretation prevails, such that dwunasta ("twelve o'clock") refers to noon and midnight is expressed as dwunasta w nocy ("twelve o'clock at night") or similar.
This is by far the prevailing form in handwritten text, but very rare in electronic communication other than official documents, owing to the difficulty of producing it in most applications.