Powershell SDK Application.Unmanage Returning Positional Error

Folks,

I'm trying to script Application.Unmanage so that we can streamline a maintenance.  We're on a fresh install of the 1.8 SDK.

This works:

     PS> Get-SwisData $swis 'SELECT NodeID, IPAddress, NextPoll FROM Orion.Nodes WHERE Vendor=@v' @{v=$Vend}

This does not:

     PS> Invoke-SwisVerb $swis Orion.APM.Application Unmanage AA:516 2014-02-11T20:00:00Z 2014-02-11T21:00:00Z false

  Invoke-SwisVerb : A positional parameter cannot be found that accepts argument '2014-02-11T20:00:00Z'.

  At line:1 char:16

  + Invoke-SwisVerb <<<<  $swis Orion.APM.Application Unmanage AA:516 2014-02-11T20:00:00Z 2014-02-11T21:00:00Z false

     + CategoryInfo          : InvalidArgument: (:) [Invoke-SwisVerb], ParameterBindingException

     + FullyQualifiedErrorId : PositionalParameterNotFound,SwisPowerShell.InvokeSwisVerb

I've tried different was to express the time, with and witthout the Z, with and without quotes, calling the invoke command and entering the fields one by one.

Any advice for me?

Thanks,

SW@REC