per the Win32::OLE::Variant docs, Date() only deals with era/year/month/day. The Time() function appears to be the one that handles converting to time with hours/minutes/seconds.

I created a spreadsheet Book1 open in Excel already, where A1 was 2024-01-26 12:34:56.789 and the Excel format "yyyy-MM-dd hh:mm:ss.000" was already applied, and it displayed right in the spreadsheet.

But I couldn't get a modified version of your code to use the $sheetdate->Time("hh:mm:ss.000") to say anything but .000, even when I used manually specified format.

Instead, I just ensured the cell's format was the "yyyy-MM-dd hh:mm:ss.000" (instead of assuming I'd done that in Excel already), and then grabbed the ->Text() from Excel, and that gave me the full string that you appear to want.

use 5.014; # strict, //, s//r
use warnings;

use Win32::OLE;
use Win32::OLE::Const 'Microsoft Excel';
use Win32::OLE::Variant;
use Win32::OLE::NLS qw(:LOCALE :DATE);

my $Excel = Win32::OLE->GetActiveObject('Excel.Application') || Win32:
+:OLE->new('Excel.Application', 'Quit');  # get already active Excel
use Data::Dump qw/dd/;
my $Book = $Excel->Workbooks('Book1');
my $Sheet = $Book->Worksheets(1);
print "Text => ", $Sheet->Range("A1")->Text();
print "Value => ", $Sheet->Range("A1")->Value();
print "{Value} => ", $Sheet->Range("A1")->Value();
my $sheetdate = Variant(VT_DATE, $Sheet->Range("A1")->Value());
print "Date => ", $sheetdate->Date();
print "Date(DATE_LONGDATE) => ", $sheetdate->Date(DATE_LONGDATE);
print "Time() => ", $sheetdate->Time();
print "Time(hh:mm:ss) => ", $sheetdate->Time('hh:mm:ss');
print "Time(hh:mm:ss.000) => ", $sheetdate->Time('hh:mm:ss.000');
$Sheet->Range("A1")->{NumberFormat} = "yyyy-MM-dd hh:mm:ss.000";
print $Sheet->Range("A1")->Text();

__END__

Text => 2024-01-26 12:34:56.789
Value => 1/26/2024 12:34:57 PM
{Value} => 1/26/2024 12:34:57 PM
Date => 1/26/2024
Date(DATE_LONGDATE) => Friday, January 26, 2024
Time() => 12:34:57 PM
Time(hh:mm:ss) => 12:34:57
Time(hh:mm:ss.000) => 12:34:57.000
2024-01-26 12:34:56.789
[download]

If you need to preserve the format in the spreadsheet, and aren't sure what it is to start with, then set my $saveformat = $Sheet->Range("A1")->{NumberFormat}; before changing it, and change it back to $Sheet->Range("A1")->{NumberFormat} = $saveformat; before you move on.