PowerShell - Was A File Created For Each Hour?

I'm attempting to create a PowerShell script that will run once every morning (via Task Scheduler) and alert someone (via Direct Send (I know how to do this and will implement said feature later)) if a file wasn't created during each hour for the day before, in order to no longer have to do this manually or when a problem arises. The filenames, as shown in the $FilePattern, all contain the date they're created, along with the time. Obviously, the logic in my Foreach loop is flawed, because $Time will always be in $Hours, but I've been scratching my head and haven't been able to figure out how I should go about this. I thought I'd be able to compare two arrays, but I'm not so sure now. I also ran across Compare-Object which seemed promising, but I couldn't quite grasp it. My apologies for the mess. My current code is as follows.

[CmdletBinding()]

param(

    [Parameter(Mandatory=$false)]
    [ValidateScript({ Test-Path $_ -PathType Container })]
    [string]
    $Path = "C:\Users\<username>\Desktop\Archive",
    $Year = [DateTime]::Today.AddDays(-1).Year,
    $Month = [DateTime]::Today.AddDays(-1).Month,
    $Day = ([DateTime]::Today.AddDays(-1).Day),
    $strMonth = ([String]$Month).PadLeft(2,'0'),
    $strDay = ([String]$Day).PadLeft(2,'0'),
    $Date = "$Year" + "$strMonth" + "$strDay",
    $FilePattern = @("850_" + $Date + "*.txt"),
    $Files = (Get-ChildItem -Path $Path -Include $FilePattern -Recurse | 
        Select-Object -ExpandProperty CreationTime | Get-Date -f "yyyyMMdd"),
    $Time = (Get-ChildItem -Path $Path -Include $FilePattern -Recurse |
        Select-Object -ExpandProperty CreationTime | Get-Date -f "hh"),
    $Hours = @(0..23)

)

Foreach ($File in $Files) {

    #if (-Not (Test-Path "$Path\$FilePattern")) {
    #    Write-Host "Warning: The file path doesn't exist!"
    #} else {
    #    Write-Host "The file path exists..."
    #}

    if ($Hours -notin $Time) {
        Write-Host "There's no file present for $Time o'clock for $Date."
    } else {
        Write-Host "There's at least one file per hour for $Date."
    }

}

Solution

Here is a shorter and probably more readable implementation:

$Path = "C:\Users\<username>\Desktop\Archive\*"
$Date = (Get-Date).AddDays(-1).ToString('yyyyMMdd')
$Hours = Get-ChildItem -Path $Path -Include "850_$($Date)*.txt" | ForEach-Object {(Get-Date $_.CreationTime).Hour} | Sort-Object | Get-Unique
for ($Hour = 0; $Hour -lt 24; $Hour++) {
    if ($Hour -in $Hours) {
        Write-Host "There's at least one file for $Hour o'clock for $Date." -ForegroundColor Green
    } else {
        Write-Host "There's no file present for $Hour o'clock for $Date." -ForegroundColor Red
    }
}

So what have I changed:

I removed the params block as I think you just used it to initialize variables, but it is designed to define parameters you want pass to a function. It does not look like you want to pass anything to that script. Correct me, if I'm wrong.
I added an asterisk (*) to your path to make the -Include parameter work (see docs). I think you added -Recurse to make it work. Use -Recurse only if your logs are located in subdirectories.
I added a way more readable and easier to understand method to create your desired timestamp.
I extracted the hours, you are looking for as numbers, so you can compare them against numbers and don't have to create an array of two digits stringified hours.
I added colors to the output to get a faster overview, if any file is missing.