I needed Powershell to plug in a single value to a .csv file that Excel would naturally convert to a Date and Time.  Here’s the bit to create that value:

# Native Powershell date and time
Get-Date

$excelDate = ((Get-Date).AddDays(1) - (Get-Date "12/31/1899")).Days
$excelTime = ((Get-Date -Format HH)/24)+((Get-Date -Format mm)/1440)+((Get-Date -Format ss)/86400)
$excelTimeStamp = $excelDate + $excelTime

$excelDate
$excelTime
$excelTimeStamp