Попробуйте это,
Declare @Get_Date date=getdate()
Select iif
(
Datepart(day,@Get_Date)<11 or Datepart(day,@Get_Date) between 26 and 31
,Format(Dateadd(day,-5,EOMONTH(@Get_Date,iif(Datepart(day,@Get_Date) between 26 and 31,0,-1))),'dd/MM/yyyy')
,Format(Dateadd(day,11,EOMONTH(@Get_Date,-1)),'dd/MM/yyyy')
) as Cutoff_Start_Date,
iif
(
Datepart(day,@Get_Date)<11 or Datepart(day,@Get_Date) between 26 and 31
,Format(Dateadd(day,10,EOMONTH(@Get_Date,iif(Datepart(day,@Get_Date) between 26 and 31,0,-1))),'dd/MM/yyyy')
,Format(Dateadd(day,25,EOMONTH(@Get_Date,-1)),'dd/MM/yyyy')
) as Cutoff_End_Date