[kh99]

You could do this:

$hour = vbdate('G');
if ($hour >= 6 AND $hour < 12)
{
    $greeting = "Good Morning";
}  
else if ($hour >= 12 AND $hour < 16)
{
    $greeting = "Good Afternoon";
}
else
{
    $greeting = "Good Evening";
}
vB_Template::preRegister('postbit_legacy', array('greeting' => $greeting));

Then in postbit_legacy, use {vb:raw greeting}

This goes by the user's local time (based on the timezone they selected in their profile). If for some reason you wanted to use the server's time for everybody, you could change vbdate to just date.

I'm kind of curious why you want to put this in postbit_legacy - wouldn't that just repeat the same greeting in every post?

Edit: after posting this I noticed that the similar threads has an existing mod something like this: https://vborg.vbsupport.ru/showthread.php?t=178745