Elasticsearch doesn't have a native concept of a "pure" date, only of instants in time. Given that, I think that it is reasonable to represent a pure date as the instant of midnight UTC on that date. If you use a different timezone then you will encounter problems as some "dates" will not be a whole number of days apart, because of occasions when the clocks go forwards or back for daylight savings.
Formatting a date as an instant without an explicit timezone is probably a bad idea, because something somewhere will have to guess what timezone to use and you may get inconsistent results if those guesses are inconsistent. Always specify the timezone, and in this case I recommend UTC as I mentioned above.
When you run an aggregation involving times, you can tell Elasticsearch what timezone to use. It defaults to UTC according to those docs.
I do not, however, know how to get Kibana to interpret these dates as you want in a visualisation. It's probably best to ask on the Kibana forum for help with that.