White background to text

Hi, I've written my own clock app and have a background image that I'm adding the time and date on top of, however, whenever the time updates the background for the text "box" becomes white and coversthat part of the image.
This is when I use 'true' as the 4th argument to drawString, if I change it to 'false' the image is shown properly but overtime the time become 88:88. I've got around it by clearing the screen and rewriting the image and text every time the screen is updated.
In the clock example (https://www.espruino.com/Bangle.js+Clock) it says that 'true', by default, only redraws the text, so I was expecting the background image to still fill the screen.
Is there something I've missed, when using drawString or do I have to stick with my workaround?

Rick

You'll have to use your workaround... Having the 4th argument draws where the text is in the background color so it'll erase the old text and put in new. But once that background has gone and been overwritten by text, Bangle.js can't figure out what it is again and replace it with what was there before...

Gordon, thank you for the quick reply and explanation.

Rick