The completion date is set by HDB, and it's not fixed.
However, should you wish to change the completion date, you will need the Seller's agreement too. Only when both parties agreed to change the completion date, that HDB will allow the change.
Alternatively, you could appoint a Power of Attorney to act for you, or if you're using bank loan, you need not attend the completion, as your conveyancing lawyer will be able to complete the case for you.
I am experienced and well-versed in both private and HDB resale transactions. I will be able to assist you in your housing plans. Please get in touch with me for a more in-depth discussion.
Should you need require further assistance in matters relating to property, please contact me at my mobile 9748 6305. I will be happy to assess and share with you the possibilities for you in the current market.