Your timestamp is already there, using $adate.
If you want something more specific, change $adate to $ctime and include the format that you want used.
Eg: $ctime($ctime,ddd mmm dd yyyy @ h:nn:ss tt)
See /help $ctime for details regarding the format used in my example and other formatting options.
To remove the message by message number, presuming that %mbnum is the message number, use /remini inimb.ini <message #>
See /help /remini
The easiest way to see the message number, is to include it as part of the message that is saved.