There's no way to set a single-day vacation message because "end date" must be > "start date".

Suppose you'll be out of office on 2019-04-15, so want to set an autoreply for that single day only (2019-04-15 00:00 --> 2019-04-15 23:59).

In the web interface, once you set 2019-04-15 as "start date", you're not allowed to set the same date as "end date", so you can only insert 2019-04-16.

Once you save the message, this is the generated .sieve script:

require ["imap4flags","fileinto","vacation","date","relational"];
if allof (header :contains "subject" "test") {
addflag "\Seen";
fileinto "INBOX.Test";
}
if allof ( currentdate :value "ge" "date" "2019-04-15", currentdate :value "le" "date" "2019-04-16" ) { vacation :days 1 :subject "Out of office" :addresses ["foo@example.org"] text:
test message
.
;
}

As you can see this vacation is active for 2 full days, 15 and 16 of April.

I think that web interface should only check "end date" >= "start date".
Additionally I'll change "start date" and "end date" field labels to explicitly state that the selected day is included in range, like:

• "Enable auto reply on (included)"
• "Disable auto reply on (included)"