I’m going to create a new extension that counts down or up seconds (or ms), but also displays the duration / time in the date format (eg: HH:mm:ss).
The special thing is that the extension does not use a clock and / or does not work with the java countdownTimer, but catch and displays the time in real time. In this way, the countdown / count up does not lose a single millisecond regardless of how long the period is.
The extension (in which I was supported by @Red_Panda with regard to some special java questions) will be extended by some functions yet, but here is a pre-release version / APK for testing:
timerFormatCountdown3.apk (3.5 MB) [updated]
See also the following comparison with the Clock component (which loses 11 sec in a 10 min countdown, on older devices may be even more):
And also the DeepHost countdown extension (which incorrectly counts down 2 seconds at the beginning and does not use a clock component, but uses Java countdownTimer) loses more than 1 second in a 10-minute countdown.
I have regularly found in my, but also in other apps, that the specified (calculated) runtimes do not correspond to the displayed ones. The deviations are considerable with longer durations. In short, the clock component may still be suitable as an egg timer for cooking eggs. 
Note: If you have any suggestions, ideas or issue with that, let me know.
