With GNU coreutils, one can pass an argument to the "set" flag like:
date -s "-3 days"
With uutils coreutils, that gives an error:
error: unexpected argument '-3' found
tip: to pass '-3' as a value, use '-- -3'
That's a bug (https://github.com/uutils/coreutils/issues/9679), but both
implementations accept --set="-3 days" just fine, so use that instead.