Convert a string to datetime in PowerShell

ParseExact is told the format of the date it is expected to parse, not the format you wish to get out.

$invoice = '01-Jul-16'
[datetime]::parseexact($invoice, 'dd-MMM-yy', $null)

If you then wish to output a date string:

[datetime]::parseexact($invoice, 'dd-MMM-yy', $null).ToString('yyyy-MM-dd')

You need to specify the format it already has, in order to parse it:

$InvoiceDate = [datetime]::ParseExact($invoice, "dd-MMM-yy", $null)

Now you can output it in the format you need:

$InvoiceDate.ToString('yyyy-MM-dd')

or

'{0:yyyy-MM-dd}' -f $InvoiceDate

You can simply cast strings to DateTime:

[DateTime]"2020-7-16"

or

[DateTime]"Jul-16"

or

$myDate = [DateTime]"Jul-16";

And you can format the resulting DateTime variable by doing something like this:

'{0:yyyy-MM-dd}' -f [DateTime]'Jul-16'

or

([DateTime]"Jul-16").ToString('yyyy-MM-dd')

or

$myDate = [DateTime]"Jul-16";
'{0:yyyy-MM-dd}' -f $myDate