Filter WordPress date functions to return relative time strings.

time_ago.php

/**
 * Filter the_time and get_the_date to return a relative string.
 * Inspired by http://www.jasonbobich.com/2011/04/10/a-better-way-to-add-time-ago-to-your-wordpress-theme/
 * Based on code from https://buddypress.trac.wordpress.org/browser/tags/2.8.2/src/bp-core/bp-core-functions.php#L1118
 *
 * @param  string $formatted The formatted time.
 * @param  string $format    The time format used.
 * @param  object $the_post  Optional - current Post object.
 * @return string            Modified formatted time.
 */
function cc_relative_time( $formatted, $format, $the_post = false ) {

	// Certain formats are likely to mean use in <time> elements and similar so don't touch them.
	$timestamp_formats = array(
		'c', // ISO 8601.
		'r', // RFC 2822.
		'U', // Unix timestamp.
	);
	if ( in_array( $format, $timestamp_formats, true ) ) {
		return $formatted;
	}

	// Get the current post object from the global if not supplied.
	if ( ! $the_post ) {
		global $post;
		$the_post = $post;
	}
	// Get the date/time for the post in GMT.
	$post_date = get_post_time( 'U', true, $the_post );

	// Get the current data/time in GMT for comparrison.
	$current_date = current_time( 'timestamp', true );

	// Language vars.
	$unknown_text = __( 'sometime', 'customcreative' );
	$right_now_text = __( 'right now', 'customcreative' );
	$ago_text = __( '%s ago', 'customcreative' );

	// Array of time period chunks.
	$chunks = array(
		YEAR_IN_SECONDS,
		30 * DAY_IN_SECONDS,
		WEEK_IN_SECONDS,
		DAY_IN_SECONDS,
		HOUR_IN_SECONDS,
		MINUTE_IN_SECONDS,
		1,
	);

	// Difference in seconds between curent time and post date/time.
	$since = $current_date - $post_date;

	// Something went wrong with date calculation and we ended up with a negative date.
	if ( 0 > $since ) {
		return $formatted;
	}

	// Find the relevant time chunk.
	for ( $i = 0, $j = count( $chunks ); $i < $j; ++$i ) {
		$seconds = $chunks[ $i ];
		// Finding the biggest chunk (if the chunk fits, break).
		$count = intval( $since / $seconds );
		if ( 0 !== $count ) {
				break;
		}
	}

	// If $i iterates all the way to $j, then the event happened 0 seconds ago.
	if ( ! isset( $chunks[ $i ] ) ) {
		$output = $right_now_text;
	} else {
		// Set output var based on $chunk.
		switch ( $seconds ) {
			case YEAR_IN_SECONDS :
				$output = sprintf( _n( '%s year ago', '%s years ago', $count, 'customcreative' ), $count );
				break;
			case 30 * DAY_IN_SECONDS :
				$output = sprintf( _n( '%s month ago', '%s months ago', $count, 'customcreative' ), $count );
				break;
			case WEEK_IN_SECONDS :
				$output = sprintf( _n( '%s week ago', '%s weeks ago', $count, 'customcreative' ), $count );
				break;
			case DAY_IN_SECONDS :
				$output = sprintf( _n( '%s day ago', '%s days ago', $count, 'customcreative' ), $count );
				break;
			case HOUR_IN_SECONDS :
				$output = sprintf( _n( '%s hour ago', '%s hours ago', $count, 'customcreative' ), $count );
				break;
			case MINUTE_IN_SECONDS :
				$output = sprintf( _n( '%s minute ago', '%s minutes ago', $count, 'customcreative' ), $count );
				break;
			default:
				$output = sprintf( _n( '%s second ago', '%s seconds ago', $count, 'customcreative' ), $count );
		}
	}

	return $output;
}
add_filter( 'the_time', 'cc_relative_time', 10, 2 );
add_filter( 'get_the_time', 'cc_relative_time', 10, 3 );
add_filter( 'the_date', 'cc_relative_time', 10, 2 );
add_filter( 'get_the_date', 'cc_relative_time', 10, 3 );

Wrap into a plugin or place in functions.php