DateTime subtraction not working in PowerShell - assignment vs. equality operator

Question

Today (2017-05-29) I am using PowerShell 5.0.10586.117 on Windows 7 Enterprise and run the following (shortened):

$dateOfLicense = \"2017-04-20\"
$dateOfToda         


        

Answer 1

To complement Martin Brandl's helpful answer:

Like many other languages - but unlike VBScript, for instance - PowerShell uses distinct symbols for:

• the assignment operator (=)
• vs. the equality operator (-eq).

This distinction enables using assignments as expressions, which is what you inadvertently did:

if (($TimeDifference) = 14) ... # same as: if ($TimeDifference = 14) ...

assigns 14 to variable $TimeDifference, as Martin explains, and, because the assignment is (of necessity, to serve as a conditional for if) enclosed in (...), returns the assigned value (the inner (...) around $TimeDifference make no difference here, however).

Therefore the (...) expression evaluated by if has value 14 - a nonzero number - it is interpreted as $true in this Boolean context.

To learn more about PowerShell's operators, run Get-Help about_Operators; to learn about how PowerShell interprets arbitrary values as Booleans in conditionals, see the bottom section of this answer.

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Finally, here's a streamlined version of your code that doesn't require intermediate variables, uses a cast to convert the string to a [datetime] instance and uses [datetime]::now, the more efficient equivalent of Get-Date (though that will rarely matter).

if (([datetime]::now - [datetime] '2017-04-20').Days -eq 14) {
  "test"
}

^{Note how "test" as a statement by itself implicitly sends output to PowerShell's (success) output stream, which prints to the console by default.
Write-Host bypasses this stream and should generally be avoided.}