Select from sql table MONTh sum

Question

My code working fine for all table records....i need to specified record for mont from combobox

 

SELECT  [radnik] as Radnik,
SUM (cast((CASE WHEN DATEPART(WEEKDAY, vrijemeprijave) in (1,7) then 0 else (CASE WHEN (DATEDIFF(SECOND, vrijemeprijave, vrijemeodjave) / 3600) > 8 THEN 8 ELSE(DATEDIFF(SECOND, vrijemeprijave, vrijemeodjave) / 3600) END) end)as float))   AS 'Radni sati',
SUM (cast((CASE WHEN DATEPART(WEEKDAY, vrijemeodjave) in (1,7) then (DATEDIFF(SECOND, vrijemeprijave, vrijemeodjave) / 3600) else(case when(DATEDIFF(SECOND, vrijemeprijave, vrijemeodjave) / 3600) > 8  then ((DATEDIFF(SECOND, vrijemeprijave, vrijemeodjave) / 3600)-8) ELSE 0 END) end)as float))   AS 'Prekovremeni'
from [prijava_radnika]
group by [radnik]

 Some help?