1930's car clock question (from a mystery)

I picked up this 1936 mystery called The Man Who Didn’t Exist for 25 cents at a used furniture store and I have found it quite good. It’s a good plot, but I am also enjoying all of the era references - the vast majority I understand - but I have come across one that I don’t remember or have not heard before. The hero gets in his car, a Ford, I think, and checks the “clock on the mirror”. Huh? What is this? There is no special mention of hanging a clock on the car mirror so is that what it is? And is it so common at the time that the author doesn’t feel the need to explain?

I consider myself knowledgable about the 20s, 30s and 40s but this one has thrown me.

Anybody?

Sounds stylish to me. And it’s available on ebay.

I’d guess it was a new accessory at the time, and thrown in to show that the car was new.

Well done Yllaria. Wonderful. I never thought it would be so common that it would be sold on ebay. So I didn’t even bother to check there. Thank you.

IIRC, mid-30s Fords, at least, had a center dash position where either a clock or a radio dial could be installed. Having a mirror clock - I seem to recall seeing sun visor clocks as well - would subtly suggest an in-dash radio, and that you owned about as ritzy a low-priced car as could be procured.