When I check my voicemail from my desk phone, why doesn't it tell me the date and time the message was left?

I agree, it is strange that it doesn't tell you the date and time the message was left. I called Dialpad support and they said there is no way to force it to give you the "message envelope" information. However, you can still view that information by looking at the message in your email, in the Dialpad app, or on the Dialpad website.