/**
 * @param  int    $emp_id Numeric employee ID, e.g. 156
 * @param  string $date   The date as 10 character string e.g. "2017-03-16"
 * @return int    The working time, in seconds. e.g. 8 hours = 28800 seconds.
 */
function get_working_hours( $emp_id, $date ) {}

$sql = sprintf(
  "SELECT punch_date FROM `%s` WHERE emp_id = %d AND punch_date LIKE '%s %%'",
  'table_name',
  (int) $emp_id,
  mysqli_real_escape_string( substr( $date, 0, 10 ) )
);
// Result SQL looks like:
// SELECT punch_date FROM `table` WHERE emp_id = 156 AND punch_date LIKE '2017-03-16 %'

// Get the results from DB, I assume you use mysqli:
$result = mysqli_query( $db_link, $sql );

if ( mysqli_num_rows( $result ) != 2 ) {
  // either invalid punches (too many or only 1), or not punched at all.
  return 0;
}

$item1 = mysqli_fetch_row( $result );
$item2 = mysqli_fetch_row( $result );

// Convert the date string to a numeric timestamp:
$time1 = strtotime( $item1[0] );
$time2 = strtotime( $item2[0] );

// Calculate and return the difference:
return abs( $time1 - time2 ); // Return value is the working time in SECONDS!