An HDFC credit card has a 30-day billing cycle. You get an extra 20 days from the date of bill generation to pay off the credit card statement's outstanding balance.
Customers are assigned billing date slots based on the card type and a few other factors, and the bank has a few fixed billing dates which cannot be changed.
Requests to change your billing cycle are not being accepted by HDFC Bank. You can, however, contact customer service and explain why you'd like to adjust your billing period. They may review and grant your request if you have a valid reason.
Additional Read: What Happens When I Miss Or Delay My Credit Card Payments?