Fading smoothness (timers)

[Techno]

whoever developed the app I am working on really should be put behind bars.

anyway
problem is, when we press a button, it should fade out the volume. this is all working with some timing issues.

now, it uses a timer (before, they were using Thread.Sleep()! Bad practice) to fade out the volume, thats all good
The volume interval is set as an application setting, so can be 6 seconds to fade out or 10 seconds or whatever

however, say if the volume is 50% and we press the button to fade, it doesnt take the full interval defined to fade out, but instead fades out earlier than the defined interval

what is the best way of tackling this so that the volume is decreased nicely/smoothly regardless of the interval but as long as it completes fully within that interval?


Now, there are 2 timers.

1) which is set for the fade interval (6 seconds)
2) the main one which is given the interval calculated based on the fade length * 1000

in the 2nd timer, the real fader, it just decreases the volume by 1 - thats all but again, regardess of the volume I want it to decrease the volume to 0 and take the entire length of the time defined to do this

does this make sense?