How to use case when function to calculate time ?

Dear All,

May i know how to use case when function to calculate the time ?

for the example , if the First_EP_scan_time is 12.30,  then must minus 30 min.  

               CASE WHEN FIRSTSCAN.EP_SHIFT <> 'R1' AND FIRSTSCAN.EP_SHIFT <> 'R2'
                THEN ROUND(CAST((DATEDIFF(MINUTE,CAST(STUFF(STUFF((CASE WHEN SHIFTCAL.EP_SHIFT = 'N1' 
                THEN CONVERT(VARCHAR(8),DATEADD(DAY,+1,LEFT(FIRSTSCAN.EP_SCAN_DATE ,8)),112) + ' ' + REPLACE(CONVERT(VARCHAR(8),DATEADD(HOUR,+0,SHIFTDESC.EP_SHIFT_TIMETO + ':00'),108),':','')
                 ELSE LEFT(FIRSTSCAN.EP_SCAN_DATE ,8) + ' ' + REPLACE(CONVERT(VARCHAR(8),DATEADD(HOUR,+0,SHIFTDESC.EP_SHIFT_TIMETO + ':00'),108),':','') END),12,0,':'),15,0,':') AS DATETIME),CAST(STUFF(STUFF(LASTSCAN.EP_SCAN_DATE,12,0,':'),15,0,':') AS DATETIME)) / 60.0 - 0.25) AS FLOAT),2) 
                 ELSE ROUND(CAST((DATEDIFF(MINUTE,CAST(STUFF(STUFF(FIRSTSCAN.EP_SCAN_DATE,12,0,':'),15,0,':') AS DATETIME),CAST(STUFF(STUFF(LASTSCAN.EP_SCAN_DATE,12,0,':'),15,0,':') AS DATETIME)) / 60.0) AS FLOAT),2) END AS OTWORK_HOUR