Should layerTime change on file save?

Question

I'm attempting to use the per-layer change times mentioned in http://blogs.adobe.com/jnack/2008/10/per-layer_metadata_comes_to_photoshop.html. The sample code at the end of the article contains the following jsx to access the layer change times:
function getLayerChangedDate(){
          var metadataStrID = stringIDToTypeID("metadata");
          var ref = new ActionReference();
          ref.putProperty(charIDToTypeID('Prpr'), metadataStrID);
          ref.putEnumerated(charIDToTypeID('Lyr '), charIDToTypeID('Ordn'),
                                                                                                                                  charIDToTypeID('Trgt'));
          var desc = executeActionGet(ref);
          if (desc.hasKey(metadataStrID)){
                    var descMetadata = desc.getObjectValue( metadataStrID );
                    var timeInSeconds = descMetadata.getDouble(stringIDToTypeID("layerTime"));
                    var d = new Date();
                    d.setTime(timeInSeconds*1000.0);
                    return d.toLocaleString();
I have a psd with two layers in it, each containing a single color rectangle. When I first open the psd if I run the above function from the ExtendScript Toolkit, they both have the same layerTime. If I change the color on one layer, its layerTime updates to the current time and the other layer's layerTime stays at the original time. All good. If I save the file, they remain correct as long as I don't close the file.
However, if I reopen the file after saving, the layerTime for both layers changes to the time at which I saved the file. As far as I can tell, the layer change time extracted by the above code always updates to the file save time, even if a layer doesn't change when saving or if it changed well before saving. Is there another way I should access layer change times? Am I misunderstanding how layer change times work?

Answer 1

I played around with this a bit more since it wasn't happening for Michael. It looks like the "update layerTime on disk but not in memory" behavior only occurs if I change some document-wide setting while not changing the layer. In my initial tests, I was changing the foreground color. The sequence was something like:
Get an initial layer changed time
Change the foreground color to change an unrelated layer's color
Check that the time for the layer from 1 hasn't changed
Save
Check that the time for the layer from 1 hasn't changed
Close and reopen the document
Check that the time for the layer from 1 updated to the time of the save
Doing some more testing now, I can cause the layerTime to be updated on save with switching the tool I'm using in addition to switching the foreground color. I'm guessing any document-wide change causes the layerTime to be changed in the psd but not in memory.