You can change your start date any time before entering the US; however, you must contact with us to confirm your new start date and there must be time for us to send you a revised I-20. It is required by US law that you enter the US with an accurate I-20 for your studies, failure to abide by this can result in problems at the US Port of Entry and/or US Embassy/Consulate and may result in denial of entry to the US.
Once you enter the US you cannot postpone your start date, there are no exceptions to this rule; however, if you want to begin studies before your start date, this may be acceptable. Please note, US law does NOT permit entry to the US more than 30 days before your I-20 start date.