Cache date for PDFs?

Why is there no date?

[micron]

I am trying to monitor the frequency that a changed PDF file is spidered by google. However, I have noticed that the cache never reports the date.

Does anyone know why google's cache doesn't report a timestamp / date for PDF entries?

Thanks.

[g1smd]

I asked Matt Cutts [mattcutts.com] about that, some time ago.

They still don't show any date.