Tried do this on a mac. I had to replace gnu grep with perl:

  perl -wnle '/(.{0}https://assets.amuniversal.com.{0,33})/ and print $1'

I had to replace wget with curl:

  curl -s https://www.gocomics.com/calvinandhobbesespanol/$x -o $z-es;

Then I realized I need gnu date, which had to come as part of coreutils:

  brew install coreutils

So date becomes gdate

  for i in {1..1000}; do x=$(gdate +'%Y/%m/%d' -d "$i days ago");
  z=$(gdate +'%Y-%m-%d' -d "$i days ago");

Hope I din't break other stuff with that.

Now it still doesn't work, but I'm giving up, hopefully I saved a little time for somebody...