Moved Duration to be initialized within model construction.

src/Discord.Net.Core/Entities/Activities/SpotifyGame.cs

@@ -54,7 +54,7 @@ namespace Discord
         /// <returns>
         ///     A <see cref="TimeSpan"/> containing the duration of the song.
         /// </returns>
-        public TimeSpan? Duration => EndsAt - StartedAt;
+        public TimeSpan? Duration { get; internal set; }
 
         /// <summary>
         ///     Gets the elapsed duration of the song.

src/Discord.Net.WebSocket/Extensions/EntityExtensions.cs

@@ -40,6 +40,7 @@ namespace Discord.WebSocket
                     Artists = model.State.GetValueOrDefault()?.Split(';').Select(x=>x?.Trim()).ToImmutableArray(),
                     StartedAt = timestamps?.Start,
                     EndsAt = timestamps?.End,
+                    Duration = timestamps?.End - timestamps?.Start,
                     AlbumArtUrl = albumArtId != null ? CDN.GetSpotifyAlbumArtUrl(albumArtId) : null,
                     Type = ActivityType.Listening,
                     Flags = model.Flags.GetValueOrDefault(),