Help > File and Directory Identifiers.

$file(filename).mtime "returns last modification time"

This in hand its a matter of looping through your directory and outputting the filenames that you want.

Code:
alias beforeapril {
  var %i = 1
  while ($findfile($mircdirtxts, *.txt,%i,var %file = $1-)) {
    if ($file(%file).mtime > 1301605200) {
      echo $nopath(%file) (Last modified $asctime($file(%file).mtime,dd/mm/yy) $+ )
    }
    inc %i
  }
}


echo -a $signature