Please note that I am not asking how to change the creation date of a bunch of files to a given date. I want to shift the date by a given amount of time, for example 47732400 seconds.
Can I accomplish this using
touch or something else in BASH?
touch only changes the creation time if the target modification time is before the original creation time.
SetFile always changes the creation time. It comes with the command line tools package that can be downloaded from Xcode's preferences or Apple's website.