The rules were posted on the website of the Cabinet of Ministers of Ukraine.
"Cabinet resolution No. 163 dated March 31, 2015, has amended Clause 2 of the procedure of extending the duration of stay and extending or shortening the period of temporary stay for foreigners and persons without citizenship in territory of Ukraine, which was introduced by Cabinet resolution No. 150 dated February 15, 2012," the posting reads.
With the adoption of the document, foreigners and persons without citizenship from countries enjoying visa-free entry into Ukraine will be able to stay in Ukraine for up to 90 days within any 180-day period, i.e. one should count 180 days backward from each day of stay (or a day of entry or passing border controls).
This means that a foreigner or a person without citizenship can legitimately stay in Ukraine without violating the migration rules if he or she spends not more than 90 days within a 180-day period in Ukraine.