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As it's stated on the website of Ministry of Foreign Affairs of the Republic of Belarus:
According to the Presidential Decree “On establishment of visa-free entry and exit of foreign nationals”, foreign citizens of 80 countries* can enter Belarus for up to 5 days** and exit from the territory of Belarus only through the State border checkpoint of the Republic of Belarus “Minsk National Airport”.
At the bottom of the page there's a clarification:
**including dates of arrival and departure.
There's even a YouTube video explaining this. So, if you arrive at 2017-09-01, you can stay till 2017-09-05 included.