Subtracting Days From Current Date
You can use the DATEADD function to do that:

CASE WHEN [RateCode] = 'AC'
      THEN DATEADD(MONTH, 
            CASE WHEN DaysPD = 30 THEN -1 
                 WHEN DaysPD = 180 THEN -6 
                 WHEN DaysPD = 360 THEN -12 
             END,
           [RateResetDate]) 
    END AS [AdjustedDate]  


Alternatively, you can just pass in the days if you want:

CASE WHEN [RateCode] = 'AC'
      THEN DATEADD(DAY, -DaysPD, [RateResetDate]) 
    END AS [AdjustedDate]