Select Next 30 days records from specified Date in SQL Server

Refer the below sample code for your reference.

-- Temporary Table
CREATE TABLE #TempTable
(EmployeeId INT
,LeaveDate DATETIME
,LeaveReason VARCHAR(30) NULL)
 GO

 -- INSERT Query
 INSERT INTO #TempTable
 SELECT 1,'2017-12-31', ''
 UNION ALL 
 SELECT 2,'2017-12-01', NULL   
 UNION ALL 
 SELECT 1,'2017-12-06', 'Sick leave' 
 UNION ALL 
 SELECT 4,'2017-12-08', NULL 
 UNION ALL 
 SELECT 18,'2017-12-12', NULL
 UNION ALL 
 SELECT 11,'2017-12-13', 'Sick leave'
 UNION ALL 
 SELECT 6,'2017-12-14', NULL 
 UNION ALL 
 SELECT 9,'2017-12-17', NULL
 UNION ALL 
 SELECT 3,'2017-12-18', 'Sick leave'
 UNION ALL
 SELECT 12,'2017-12-25', NULL
 UNION ALL 
 SELECT 16,'2017-12-27', 'Sick leave'
 UNION ALL 
 SELECT 17,'2018-01-02', NULL
 UNION ALL 
 SELECT 15,'2018-01-05', 'Sick leave'
 UNION ALL 
 SELECT  11,'2018-01-11', NULL
 UNION ALL 
 SELECT  13,'2018-01-12', 'Sick leave'
 UNION ALL
 SELECT  11,'2018-01-15', NULL
 UNION ALL 
 SELECT   4,'2018-01-16', NULL
 UNION ALL 
 SELECT   2,'2018-01-17', 'Sick leave'
GO

-- Test query to check the records in thirty days
DECLARE @SearchDate DATETIME
SET @SearchDate = '2017-12-16 00:00:00.000'
SELECT * FROM #TempTable
WHERE LeaveDate BETWEEN @SearchDate 
AND  DATEADD(DAY,30,@SearchDate) 
ORDER BY LeaveDate

Output