How about creating a device for this?
Most likely you will already know what the hour is. If you were in a torpor - at work for example

- you can get away with it by saying "x minutes" past or "x minutes" to.
So the only thing you need is the minute hand on a watch.
I wonder if you could buy an inexpensive wrist watch, remove the mechanism from it, and attach a shirt button to it on which you would indent a mark.
Then the watch mechanism would be hidden behind your shirt, leaving the button to protrude.
Or could the same principle be applied to a circular lapel pin, with the mechanism hidden behind the lapel?