Yes, whenever you decide the launch date, just assign that date to an embedded data. In the header under look & feel tab, just pipe in this date, to get reflected in the header of all page.

rondev the key question is what is the pipeline code for the launch date? date=${launchdate}? Qualtrics support say they do not have this feature built in.

rondevthe key question is what is the pipeline code for the launch date? date=${launchdate}? Qualtrics support say they do not have this feature built in. - Hence I am saying to assign that Embedded data every time before launching.
OR pipe in the current date in the survey link/ URL and capture that in survey flow ED

rondev "OR pipe in the current date in the survey link/ URL and capture that in survey flow ED" piped current date changes over days. I want to have the launched time, a fixed date. You mean current date in the survey link/URL is the survey launching date? How do you do that ${date://CurrentTime/TL} changes over days. you mean something like ${date://CurrentTime/TL && link/URL}?

rondev I succeeded in following you this suggestion "assign that Embedded data every time before launching". So now I have an embedded data LaunchDate displayed as "Thursday, April 16". I actually want to show the subjects the date 7 days after the launched date which is "Wednesday, April 22". Do you know how to display "Wednesday, April 22"? I tried $e{${e://Field/LaunchDate}+7 } but it does not work.

I know this would be too manual but similarly you can create one more embedded and assign respective date. Else you can use JS to parse LaunchDate embedded data and add 7 days to it.