How can Time be made to go backwards?

Question

I would like to make a countdown timer, but cannot find a reliable method to make the DateList[] function reverse.

The not so robust code for the Digital Clock with the display of seconds follows:

digit[i_, xoffset_] := Module[{params},
   params = Part[segmentParams, segmentPieces[[i + 1]]];
   params = Map[# + {0, xoffset, 0, 0} &, params];
   Graphics[makeSegment /@ params]];

makeSegment[params_] := 
 Translate[
  Rotate[Polygon[{{0, 0}, {params[[4]], 0}, {params[[4]] + 1/4, 
  1/4}, {params[[4]], 1/2}, {0, 1/2}, {-1/4, 1/4}}], 
   params[[1]] Degree], {params[[2]], params[[3]]}]
segmentParams = {{0, 0, 1.2, 1}, {90,-.6, .6, .6}, {90,1,.6,.6}, {0, 0, 0, 1},
     {90,-.6,-.6,.6}, {90, 1,-.6,.6}, {0, 0,-1.2, 1}};
segmentPieces = {{1, 2, 3, 5, 6, 7}, {3, 6}, {1, 3, 4, 5, 7}, {1, 3, 4, 6, 7},
 {2, 3, 4, 6}, {1, 2, 4, 6, 7}, {1, 2, 4, 5, 6, 7}, {1, 3, 6}, 
 {1, 2, 3, 4, 5, 6, 7}, {1, 2, 3, 4, 6, 7}};
timer = Dynamic@Refresh[Show[{digit[Floor[DateList[][[6]]/10], 12], 
 digit[Mod[Floor@DateList[][[6]], 10], 15]}, ImageSize -> 26], UpdateInterval -> 1]

I say not so robust because after it's running, it has a tendency to stop. Not sure why yet, but none the less I would like to make it run backwards. I can use: RunScheduledTask to begin the timer and set an alarm but would like to show the time counting down.

Answer 1

Replace your timer = .. by

tf = IntegerPart[AbsoluteTime[] + 60];

Dynamic@Refresh[
  Show[{digit[#[[-2]], 12], digit[#[[-1]], 15]}, ImageSize -> 26] &@
   IntegerDigits[1000 + tf - IntegerPart@AbsoluteTime[]],   UpdateInterval -> 1]

Edit

To force the countdown to stop after reaching zero, you could do something like:

tf = IntegerPart[AbsoluteTime[] + 10];
expired = False;
Dynamic@Refresh[
  If[(! expired ) && # != {1, 0, 0}, 
     Show[{digit[#[[-2]], 12], digit[#[[-1]], 15]}, ImageSize -> 26],
     (expired = True; digit[0, 12])] &@
              IntegerDigits[100 + tf - IntegerPart@AbsoluteTime[]], 
  UpdateInterval -> 1]