WordPress function to change post date to Exif date.

sjf_deh_exif_add.php

<?php

/**
 * Hooks onto an attachment post just after it's created and alters the post date to reflect the EXIF time.
 * 
 * @param  int $attachment_ID The ID of the post we're altering.
 * @return int The ID of the post we're altering.
 */
function sjf_deh_exif_add( $attachment_ID ) {
    
    // Grab the exif date for this image
	$time = sjf_deh_get_exif_date( $attachment_ID );
	
	// If we can't grab the exif date, bail.
	if( !$time || empty( $time ) ) { return $attachment_ID; }

	// Build up an array of post attributes for updating the post.
	$update = array(
		'ID' => $attachment_ID,
		'post_date' => $time,
	);

	// Update the post.
	wp_update_post( $update );

    return $attachment_ID;

}
add_filter( 'add_attachment', 'sjf_deh_exif_add' );