Given a date in the timezone of the site, returns that date in UTC.

Requires and returns a date in the Y-m-d H:i:s format.
Return format can be overridden using the $format parameter.

function get_gmt_from_date( $string, $format = 'Y-m-d H:i:s' ) {
	$datetime = date_create( $string, wp_timezone() );

	if ( false === $datetime ) {
		return gmdate( $format, 0 );
	}

	return $datetime->setTimezone( new DateTimeZone( 'UTC' ) )->format( $format );
}