Powershell get date add Weeks and show the remaining time left?

I am trying to create or get the result for the date and days with the Weeks, but somehow Powershell cannot do it for me:

(Get-Date '29/2/2020').addweeks(18)

The error I have received is like:

(New-TimeSpan -Start "$Get-Date" -End "(Get-Date '29/2/2020').addweeks(18)").ToString("dd' Days 'hh' Hours 'mm' Minutes 'ss' Seconds'")
Method invocation failed because [System.DateTime] does not contain a method named 'addweeks'.
At line:1 char:1
+ (Get-Date '29/2/2020').addweeks(18)
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + CategoryInfo : InvalidOperation: (:) [], RuntimeException + FullyQualifiedErrorId : MethodNotFound
New-TimeSpan : Cannot bind parameter 'Start'. Cannot convert value "-Date" to type "System.DateTime". Error: "String was not recognized as a valid DateTime."
At line:3 char:22
+ (New-TimeSpan -Start "$Get-Date" -End "(Get-Date '29/2/2020').addweek ...
+ ~~~~~~~~~~~ + CategoryInfo : InvalidArgument: (:) [New-TimeSpan], ParameterBindingException + FullyQualifiedErrorId : CannotConvertArgumentNoMessage,Microsoft.PowerShell.Commands.NewTimeSpanCommand

The output expected is:

Days, dd MM YYYY hh:mm AM/PM
"dd' Days 'hh' Hours 'mm' Minutes 'ss' Seconds'" remaining until the date entered.

1 Answer

New-TimeSpan's -Start and -End parameters expect arguments of type [DateTime], so you have to ensure your Get-Date commands get evaluated before being passed of to New-TimeSpan.

$now = Get-Date
$18WeeksFromLeapDay = (Get-Date '29/2/2020').AddDays($(18 * 7))
$timespan = New-TimeSpan -Start $now -End $18WeeksFromLeapDay

You can also omit the -Start argument completely and it will default to "right now" anyway:

$timespan = New-TimeSpan -End (Get-Date '29/2/2020').AddDays($(18 * 7))

I'd strongly suggest consulting the about_Parsing and about_Parameters help files to get a better understanding of how PowerShell evaluates command expressions and parameter arguments