Since you are using SQL 2012, let's use some of the new features:

First put some test data into a table to run from. You really should have provided some test data for us in readily consumable format, like I did below. Please see the first link in my signature for how to do this.

DECLARE @Customers TABLE (

CustomerID INTEGER,

ApptDt DATE,

PRIMARY KEY (CustomerID, ApptDt)

);

INSERT INTO @Customers (CustomerID, ApptDt)

VALUES (1, '2014-05-01'),

(1, '2014-06-15'), -- 45 days between appts

(2, '2014-06-01'),

(2, '2014-06-28'),

(3, '2014-03-20'),

(3, '2014-10-25');

-- create some variables

DECLARE @StartDate DATE = '2014-01-01',

@EndDate DATE = '2014-08-31';

WITH cte AS

(

-- Get the customers and their first appointment within the specified date range.

SELECT CustomerID, MIN(ApptDt) AS FirstApptDt

FROM @Customers

WHERE ApptDt >= @StartDate AND ApptDt <= @EndDate

GROUP BY CustomerID

), cte2 AS

(

SELECT t2.CustomerID,

t2.FirstApptDt,

-- get the next appointment for this customer

LEAD(ApptDt, 1, NULL) OVER (PARTITION BY t1.CustomerID ORDER BY t1.ApptDt) AS NextApptDate,

-- get the row number for this row

ROW_NUMBER() OVER (PARTITION BY t1.CustomerID ORDER BY t1.ApptDt) AS RN

-- join the subquery to the table

FROM @Customers t1

JOIN cte t2

ON t1.CustomerID = t2.CustomerID

)

-- now add the number of days between the two dates,

-- and filter for just the first row and for appointments > 30 days.

SELECT cte2.CustomerID,

cte2.FirstApptDt,

cte2.NextApptDate,

ca.DaysBetweenAppts

FROM cte2

CROSS APPLY (VALUES (DATEDIFF(DAY, cte2.FirstApptDt, cte2.NextApptDate))) ca(DaysBetweenAppts)

WHERE RN = 1

AND ca.DaysBetweenAppts >= 30

Does this handle what you're looking for?