You cannot call a method on a null-valued expression.
At C:commonSCRIPTSPASSWORD_EXPIRY_CUSTOM.ps1:9 char:116
+ ... ires | where { $_.PasswordExpires -gt $null -and `
+                    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : InvalidOperation: (:) [], RuntimeException
    + FullyQualifiedErrorId : InvokeMethodOnNull

$date = Get-Date

$FirstWarning = $date.AddDays(2).ToShortDateString()
$SecondWarning = $date.AddDays(7).ToShortDateString()
$LastWarning = $date.AddDays(15).ToShortDateString()



$Users = Get-QADUser -SizeLimit 0 -SearchRoot 'mydomain.com/users' -IncludedProperties PasswordExpires |
             Where {
                 $_.PasswordExpires -gt $null -and `
                ($_.PasswordExpires).ToShortDateString() -eq "$FirstWarning" -or `
                ($_.PasswordExpires).ToShortDateString() -eq "$SecondWarning" -or `
                ($_.PasswordExpires).ToShortDateString() -eq "$LastWarning"
             }

$Users = Get-QADUser -SizeLimit 0 -SearchRoot 'mydomain.com/users' -IncludedProperties PasswordExpires | where { $_.PasswordExpires -ne $null -and `
( `
  ($_.PasswordExpires).ToShortDateString() -eq "$FirstWarning" -or `
  ($_.PasswordExpires).ToShortDateString() -eq "$SecondWarning" -or `
  ($_.PasswordExpires).ToShortDateString() -eq "$LastWarning" `
) `
}