You want to calculate a past or future timeday from a given one.
#!/usr/bin/env raku
use v6;
# the past using whole days
my $wall = Date.new("1981-06-17");
say ($wall-28);
# the future with datetimes
my $dt = DateTime.new("1981-06-17T20:00:00Z");
say $dt.later(:week(2));
# vim: expandtab shiftwidth=4 ft=perl6