You could use the LAG() Windows Function which retrieves the value of the previous row. LEAD() retrieves the value on the next row. Something like;

DECLARE @StartDate AS DATETIME

SET @StartDate =...

if you are working with SQL Server 2016 then there is a new function called STRING_SPLIT ( string , separator )

I would probalby do it like this

WITH CTE AS (

SELECT E.EmployeeNumber, MAX(E.[Timestamp]) AS DateLastAdded

FROM dbo.Employees AS E

GROUP BY E.EmployeeNumber

)

SELECT Emp.EmployeeNumber, Emp.RagRating, Emp.[Timestamp],

DATEDIFF(d, Emp.[Timestamp], GETDATE()) AS DateSinceAdded

FROM...