Macro for setting default date format. More info: http://craftsnippets.com/articles/working-with-dates-in-craft-cms-templates

default_date_format.twig

{%- macro defaultDateFormat(date, additionalAttributes) -%}
{# v1 #}
{% if date is defined %}
{# settings #}
{% set format = 'medium' %}
{# logic #}
{% set attributes = {
	text: date|date(format),
	datetime: date|date('yy-m-d')
} %}
{% if additionalAttributes is defined and additionalAttributes is not iterable %}
	{% set attributes = attributes|merge({class: additionalAttributes}) %}
{% elseif additionalAttributes is iterable %}
	{% set attributes = attributes|merge(additionalAttributes) %}
{% endif %}
{{tag('time',attributes)}}
{% endif %}
{%- endmacro -%}