Looping through all subfolders, zipping contents - powershell

Question

I'm trying to zip all files in all subdirectories that are older than 31 days.
The following code mainly works, but does some things wrong. It creates new folders and zips them, as desired.
But, it doesn't seem to move the old files into the newly-created directories befoee they are zipped. The line Get-ChildItem -Exclude *.zip | ? {$_.LastWriteTime -lt (Get-date).AddDays(-31) -and -not $_.psIsContainer} | Move-Item -destination $newpath should do that, but doesn't. When I try to run it on its own in the console, it works fine however.
Also, in a tree of folders, this code won't work on the outer edges of the tree. What I mean is that the last folder will not have a new folder created within it.
Does anyone know where I'm going wrong?
function zipfiles ($path,$name){
$date = "$((Get-Date).ToString('yyyy-MM-dd'))"
$newpath = "$path$date"
New-Item -ItemType Directory -Path $newpath
Get-ChildItem -Exclude *.zip | ? {$_.LastWriteTime -lt (Get-date).AddDays(-31) -and -not $_.psIsContainer} | Move-Item -destination $newpath
[Reflection.Assembly]::LoadWithPartialName( "System.IO.Compression.FileSystem" )
$src_folder = $newpath
$destfile = "$newpath.zip"
$compressionLevel = [System.IO.Compression.CompressionLevel]::Optimal
$includebasedir = $false
[System.IO.Compression.ZipFile]::CreateFromDirectory($src_folder,$destfile,$compressionLevel, $includebasedir)
Remove-Item $newpath -recurse
}
Get-ChildItem -Recurse -Directory | ForEach-Object { zipfiles($($_.FullName),$($_.Name))}