Actually date range labela are like this:

start date: "Enable auto reply on"
end date: "Disable auto reply on"

Most of my users misunderstand the meaning of the end date field; they interpret it as "the first day back to work" instead of "the last day of vacation"...

I suggest to improve them like this:

start date: "Enable auto reply on (included)"
end date: "Disable auto reply on (included)"

or maybe add a new note under the two fields, like:
"NOTE: both dates included"