diff --git a/bin/ssa-retime b/bin/ssa-retime
index ed9cccc..93a57ca 100755
--- a/bin/ssa-retime
+++ b/bin/ssa-retime
@@ -160,6 +160,17 @@ if ($mode eq 'framerate') {
     }
     print "-" x 30, "\n";
   }
+  # video: 40s 47s    64s
+  # |-------*---*------*---------|
+  #         ^   ^      ^-- b (-3.0s)
+  #         |   `--------- t ( ?.?s)
+  #         `------------- a ( 0.5s)
+  # ----------------------------------------------------
+  # pct = (47.0 - 40.0) / (64.0 - 40.0) => 0.29 (29%)
+  # abs = (0.5 ~~ -3.0)                 => -3.5
+  # mod = -3.5 * 0.29                   => -1.01
+  # t  += 0.5 + -1.01                   => -0.51s
+  # ----------------------------------------------------
   # real retime
   $ssa->sort_events;
   foreach my $e (@{ $ssa->events }) {