For a left-sided match you can get around the automatic adding of '%'
‘s by WP by using regular expression RLIKE
:
'meta_value' => '^' . preg_quote( $today ),
'meta_compare' => 'RLIKE'
Similarly for right-sided match:
'meta_value' => preg_quote( $today ) . '$',
'meta_compare' => 'RLIKE'