windows - Powershell LastWriteTime is incorrect

I am running the following script to create directories based on the first 9 characters in the filenames in the directory and then moving te files into these directories based on their filenames.

dir | %{ 
    $id = $_.Name.SubString(0,9); 
    if(-not (Test-Path $id)) {mkdir $id}; 
    mv $_ "$id\$_";}

Now, what I have found is that after running the script the folders are being marked with a LastWriteTime several years in the past.

Here is the output from Powershell

Mode                LastWriteTime     Length Name
----                -------------     ------ ----
d----        13/04/2006   7:25 PM            XXX095_00
d----        13/04/2006   7:29 PM            XXX285_0_

Can anyone tell me where this LastWriteTime is coming from?

Moving a file doesn't appear to count as a write. I guess that's because you aren't actually writing to the contents of the file. But you can update the LastWriteTime yourself. Try this:

dir | %{ 
    $id = $_.Name.SubString(0,9); 
    if(-not (Test-Path $id)) {mkdir $id};
    $_.LastWriteTime = Get-Date;           # <---- Added line.
    mv $_ "$id\$_";}

(You don't need the semicolons, by the way, when your statements are on separate lines.)