Cognizant — HR not responding to change date of joining in offer letter

Address: Manyata techpark Bangalore

I have received the offer letter from congnizant however the date of joining us given as the same date as my last working day in my current organization. I have been sending multiple emails to the recruiter and tried calling multiple times but no response. My date of joining is 21/06/2022 which needs to be changed. Need to HR's help on this ASAP