`touch` has an adjust option (`-A`), but that is sadly limited to 99 hours, 99 minutes, and 99 seconds. But repeated applications can get you what you want:

in Bash:

 # assume a year is 365 days
 for day in {1..365}; do 
 touch -A 240000 folder/* 
 done

 # on a mac with Xcode cmdline tools installed, 
 # set creation time to mtime:
 for f in folder/* ; do
 SetFile -d "$(GetFileInfo -m $f)" $f
 done