I have analysed and fixed the issue.
It was caused by a totally unexpected buggy behaviour of your outdated Magento version (2.2.6).
Magento developers tried to fix issues of various dates formatting in different locales and did it in a straightforward and incorect way: they just add an extra day to the special promotion end date ($toTimeStamp += 86400;):
https://github.com/magento/magento2/blob/2.2.6/lib/internal/Magento/Framework/Stdlib/DateTime/Timezone.php#L275-L278
I have fixed it now.
upwork.com/messages/rooms/room_8e141f0c39ea3e5091cd334db37aaef0/story_8b1ee9633f632773f154d3cb81416f95