diff --git a/CHANGELOG.md b/CHANGELOG.md
index 6755fce..9a94f2c 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -9,6 +9,10 @@ Each release is tagged in Git and on the Docker repository
 ## `v0.6.3` (in progress)
 
 *   Compile fix for nightly rust 2021-03-14 and beyond.
+*   Fix incorrect `prev_media_duration_90k` calculation. No current impact.
+    This field is intended to be used in an upcoming scrub bar UI, and when
+    not calculated properly there might be unexpected gaps or overlaps in
+    playback.
 
 ## `v0.6.2`
 
diff --git a/server/db/db.rs b/server/db/db.rs
index df5200a..3228e30 100644
--- a/server/db/db.rs
+++ b/server/db/db.rs
@@ -826,7 +826,7 @@ impl LockedDatabase {
             Some(s) => {
                 let l = s.lock();
                 r.prev_media_duration =
-                    l.prev_media_duration + recording::Duration(l.wall_duration_90k.into());
+                    l.prev_media_duration + recording::Duration(l.media_duration_90k.into());
                 r.prev_runs = l.prev_runs + if l.run_offset == 0 { 1 } else { 0 };
             }
             None => {
diff --git a/server/db/schema.sql b/server/db/schema.sql
index 97f5981..e37cb43 100644
--- a/server/db/schema.sql
+++ b/server/db/schema.sql
@@ -192,7 +192,8 @@ create table recording (
   wall_duration_90k integer not null
       check (wall_duration_90k >= 0 and wall_duration_90k < 5*60*90000),
 
-  -- TODO: comment.
+  -- The media-time duration of the recording, relative to wall_duration_90k.
+  -- That is, media_duration_90k = wall_duration_90k + media_duration_delta_90k.
   media_duration_delta_90k integer not null,
 
   video_samples integer not null check (video_samples > 0),