Two identical strings are "not equal". Why?

Im trying to figure out why these two strings are not equal. Is there a better operator I should use?

$lastupdated = invoke-webrequest -uri $source | Out-String -Stream | Select-String Last-Modified

$storeddate = get-content C:\windows\file.txt

$lastupdated -ne $storeddayed
True

Indeed means that the values aren't identical even their basic properties (display values) appear to be the same.

PowerShell is a loosely typed programming which lets you "convienently" compare two values of a different type. Try this:

$lastupdated.GetType()
$storeddayed.GetType()

I suspect that the $storeddayed variable is actually a string as the name implies that it has been stored were it is has likely lost its fidelity of the actual datetime structure as it has either been converted automaticaly or "manually" as you do in your own answer with the ToString() method.
If you compare two different types the objects. The object at the RHS (right hand side) dictates the conversation for the comparison (this generally counts for any other operator).
This means that you might just convert the right side (or both sides) to a string (using double quotes):

"$lastupdated" -ne "$storeddayed"

Or simply swap the operants in the operation to get what you expected:

$storeddayed -ne $lastupdated

Also note that in case the operants ($storeddayed and $lastupdated) are both of the datetime structure, not all detailed properties are displayed by default (think of the timezone information and e.g. the millisecond property).

In other words, if you give the commands in one go:

$lastupdated = Get-Date()
$storeddayed = Get-Date()

They might look the same as you display them (as string) but they actually differ in time:

$lastupdated.Millisecond
$storeddayed.Millisecond

Root Cause

Based on how you're initialising $storeaddate...

$storeddate = get-content C:\windows\file.txt

... I'm going to assume your file has just got a single line of text with something like this in it..

Last-Modified     Web 17, May 2023 23:27:24 GMT

... which makes it a [string] instance.

Whereas $lastupdated is initialised like this:

$lastupdated = invoke-webrequest -uri $source | Out-String -Stream | Select-String Last-Modified

which means it's a Microsoft.PowerShell.Commands.MatchInfo object because that's what Select-String returns.

You can check that with this:

$lastupdated.GetType().FullName

# Microsoft.PowerShell.Commands.MatchInfo

and you can see more details with this:

$lastupdated | fl *

# IgnoreCase : True
# LineNumber : 1
# Line       : Last-Modified     Web 17, May 2023 23:27:24 GMT
# Filename   : InputStream
# Path       : InputStream
# Pattern    : Last-Modified
# Context    :
# Matches    : {0}

The reason youc $storeddate -ne $lastupdated always returns $true is because a [string] and a [matchinfo] are never equal.

Workaround

If you want to get a textual (i.e. [string]) representation of the return value from Select-String you can do this:

$lastupdated = invoke-webrequest -uri $source `
    | Out-String -Stream `
    | Select-String Last-Modified `
    | select-object -ExpandProperty "Line"
    # ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ - extract the "Line" property value

$lastupated

# Last-Modified     Web 17, May 2023 23:27:24 GMT

(note the select-object -ExpandProperty "Line" which extracts the value of the Line property from the matchinfo object)

And now $lastupdated really is a [string] as opposed to a MatchInfo and you should find $storeddate -ne $lastupdated will correctly return $false when they're the same.

Last-Modified Header

Having said all of that, I'm going to take a bit of a leap and guess that you're really actually after the Last-Modified response header from your call to invoke-webrequest which you can get a lot easier if you do this:

$response = invoke-webrequest -uri $source

$lastupdated = $response.Headers["Last-Modified"]

$lastupdated

# Web 17, May 2023 23:27:24 GMT

which won't be affected by differences in brittle stringifying of the response before you extract the Last-Modified using select-string.

And, if you're using the Last-Modified header to decide whether a resource has been updated, you might as well use the HEAD method as well to avoid downloading the entire resource content, since that's what you're trying to avoid doing in the first place if it's already up to date:

$response = invoke-webrequest `
   -Uri    "www.google.com" `
   -Method "HEAD" `
   -UseBasicParsing

$lastupdated = $response.Headers["Last-Modified"]

$lastupdated

# Web 17, May 2023 23:27:24 GMT

I added the .ToString() when saving the string to a file and now my operator seems to be working.

$lastupdated.ToString() | out-file c:\windows\file.txt