var palindates = gather {
    for y in (2020 .. 9999) {
        var (m, d) = Str(y).flip.last(4).split(2)...
        with ([y,m,d].join('-')) {|t|
            take(t) if Date.valid(t, "%Y-%m-%d")
        }
    }
}

say "Count of palindromic dates [2020..9999]: #{palindates.len}"

for a,b in ([
    ["First 15:", palindates.head(15)],
    ["Last 15:",  palindates.tail(15)]
]) {
    say ("\n#{a}\n", b.slices(5).map { .join("   ") }.join("\n"))
}