Discrepancy in relieving letter

Hi sir, I have concern in my relieving letter. I collected my relieving letter from Current company((Magna Infotech). The start and end dates are specified as 5-sep-2013 and 17-Oct-2014. But my Previous relieving letter from Technosoft shows that Relieving date as 11-Sept-2013. This is causing the issue for my new offer. As HR is telling that BG verification will fail in this regard. When I was joining Magna, I told the same to recruiter that I will be relieved from previous company(Technosoft -- I was in bench from 12th July 2013) on 11th sept 2014(My bad it was in telephonic discussion and hence does not have any proof). As it was an urgent requirement for Sapient, I have immediately joined Magna on 5th Sept 2014. I didn't get any problem with Magna until I shared the documents with CapGemini and SLK software. They are telling it as Dual employment. Now I am not getting any help from Magna for the modification of start date. Please advise me how to proceed legally? Thanks, Jagan.T