I think this is what you need (I added a [>= 30 days] category just for the heck of it):
SELECT SUM(ISNULL(CASE WHEN date_diff < 10 THEN 1 END, 0)) AS [< 10 days], SUM(ISNULL(CASE WHEN date_diff >= 10 AND date_diff < 30 THEN 1 END, 0)) AS [>= 10 & < 30 days], SUM(ISNULL(CASE WHEN date_diff >= 30 THEN 1 END, 0)) AS [>= 30 days] FROM YourTable
