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