If you are building a Wordpress theme then there will a time when you want to display the date for the current post, luckily for you this is very easy with a Wordpress function the_date(). The the_date function will return the date of the post or the set of posts if published on the same date. This function has the option of passing in 4 parameters.
HTML to place before the date
HTML to place after the date
Boolean if to echo the date or not.
<?php the_date( $format, $before, $after, $echo ); ?>
If you are using the Wordpress loop to display your posts then you should be using the get_the_date() function. The difference is get_the_date() will always return the date of the current post. This doesn't have any parameters so using it is very simple.
<span class="entry-date"><?php echo get_the_date(); ?></span>
If you want to display the date like Twitter does so 2 days ago, 1 day ago, Wordpress has a function where you can display the difference of 2 dates. By using the human_time_diff() function.
<?php human_time_diff( $from, $to ); ?>
If you don't supply it with a to date it will default to now. To display 2 days ago use the following snippet.
<?php echo human_time_diff( get_the_time('U'), current_time('timestamp') ) . ' ago'; ?>