Filter the *displayed* time of posts in WordPress to move them into the past/future.

time_machine.php

/**
 * Filter the_time function to show posts as being from another date/time.
 *
 * @param  string $formatted The formatted time.
 * @param  string $format    The time format used.
 * @return string            Modified formatted time.
 */
function cc_time_machine( $formatted, $format ) {
	$offset = get_option( 'cc_time_offset' ); // Offset in days, can be negative.
	if ( $offset ) {
		global $post;
		$timestamp = get_post_time( 'U', true, $post ); // UTC timestamp with GMT offset included.
		$adjusted = $timestamp + ( (int) $offset * DAY_IN_SECONDS );
		return date( $format, $adjusted );
	}
	// If option doesn't exist, then just return it unmodified.
	return $formatted;
}
add_filter( 'the_time', 'cc_time_machine', 10, 2 );

Wrap this into a plugin or just drop it into your functions.php
Remember you need to create / manage the option for $offset, or remove get_option() and replace it with a value, e.g. '-15'.