Rename files to get the year, month and day of the creation in front of the file?

pspBugs 

Here's a three-line example on how you can do this. I've gone with the assumptions that:

1. The prefix date format should be "yyyyMMdd_";
2. You do indeed want the creation date used in the prefix, not the last modified date.

$ParsedDate = [datetime]::MinValue;
$Path = "D:\Data\SomePath";
Get-ChildItem -File -Path $Path -Recurse |
    Where-Object { (-not [regex]::IsMatch($_.Name, "^\d{8}_", [System.Text.RegularExpressions.RegexOptions]::IgnoreCase)) -or (-not [datetime]::TryParseExact($_.Name.Substring(0, 8), "yyyyMMdd", [cultureinfo]::CurrentCulture, [System.Globalization.DateTimeStyles]::None, [ref] $ParsedDate)) } |
        ForEach-Object { Rename-Item -Path ($_.FullName) -NewName "$($_.CreationTime.ToString("yyyyMMdd"))_$($_.Name)"; }

Cheers,

Lain