MySQL select only where Timestamp is from last 10 days

You can use DATEDIFF or, as already posted, DATE_SUB. Also, I suggest not using reserved words like "Date" for column names. Example for your code:

WHERE DATEDIFF(NOW(), `Date`) < 10

SELECT *
FROM Comments
WHERE (City = '$city2') AND (`Date` > DATE_SUB(now(), INTERVAL 10 DAY));

Note: calling a column 'Date' is poor practice, since it's a reserved word.