Just in case you want a custom solution, you could write a Jekyll plugin to format a date as you want, like this (this example is for Italian dates):
module Jekyll
    module ItalianDates
        MONTHS = {"01" => "gennaio", "02" => "febbraio", "03" => "marzo",
                "04" => "aprile", "05" => "maggio", "06" => "giugno",
                "07" => "luglio", "08" => "agosto", "09" => "settembre",
                "10" => "ottobre", "11" => "novembre", "12" => "dicembre"}
        # http://man7.org/linux/man-pages/man3/strftime.3.html
        def italianDate(date)
            day = time(date).strftime("%e") # leading zero is replaced by a space
            month = time(date).strftime("%m")
            year = time(date).strftime("%Y")
            day+' '+MONTHS[month]+' '+year
        end
        def html5date(date)
            day = time(date).strftime("%d")
            month = time(date).strftime("%m")
            year = time(date).strftime("%Y")
            year+'-'+month+'-'+day
        end
    end
end
Liquid::Template.register_filter(Jekyll::ItalianDates)
Just save this to a file like _plugins/italian_dates.rb and use it as you need in templates:
<time datetime="{{page.date | html5date}}">{{page.date | italianDate}}</time>