The clock on the N9 is called a low power mode (or something on the theme). Basically, only the pixels showing the digits are on, the rest of the display is off. It is a feat possible because the N9 uses an OLED display.
Jolla uses an LCD. That is a different technology which does not offer that possibility. It is either all on or all off. In theory one could leave the display on but the backlight off to save power, but I am not sure about the practicality of such a solution. You probably would not be able to read the clock.
There is an amateur project to add a small OLED display to the back of the Jolla. Look it up in this forum if interested. Something like OLED TOH.
It is possible. It's just a matter of how much battery it will drain. Remember old Nokias? They all had that clock even though that was a simple LCD, now Lumias have this clock no matter the screen type. We just have to figure out how to do it right.
Here's an article about glance screen on the Lumia 920 (4.5" IPS 1260x768). It's not as bright as on AMOLED models. Apparently it's done by using a low brightness level for the backlight.
