Same problem and steps to reproduce as at T12839: Can't create one day event. (Normally I would reopen that task, but I don't have the permissions to do so)
Version:
phabricator
a6b550ba0394284441ee55d11e276a05eb568ad9 (Tue, Jul 11)
arcanist
165df12046e58390f0d00f4cf3e1aa1ab0eb1365 (Fri, Jul 7)
phutil
0a4487d37cd72b3b91ac332377f2b12d4e5a2543 (Fri, Jun 23)
(current master)