Subtract 30 days from due date

Hi ,

I have a vendor payment Date format yyyymmdd 20151231 ( NVARCHAR - 8 length ) .

If the vendor will pay bfore 30 days ,company will give some offer.

So I need the findout the gross payment date which is ( actual payment Date - 30 days ). it should be 20151231- 30 = 20151201

I have created one calculated column with formula adddays(date,int) , adddays("paymentDate",-30) .

This function is not working . Please suggest any other option is there.

I can able to add 30 days which is working fine but not able to deduct 30 days from the paymentDate.

Regards,

Kamruz