Ok so saddogimmo dug trough the game's code and found the score's formula. The game runs at 50 FPS:

The bonus is calculated: 2000-floor(20*(CurrentFrames/TimeBorders)). If this results negative, the bonus is zero.

TimeBorders is the list of all maximum time/2 you can do on a level. Ex. on level 1 the maximum time is 4 seconds, so TimeBorders is 2.

Also, the seconds in-game get rounded to the nearest integer.

Score calculator: https://www.desmos.com/calculator/bf2usfk8bq

The bonus formula I've provided here is a mathematical simplification of the source code, thus it is not the original; however, it yelds the exact same result. Here is the full code for the bonus score calculation:

private function calculateBonus() : void

{

var *loc1*:Number = NaN;

if(getCurrentTime() <= timeBorders[_parent._currentLevel - 1] * 2)

{

*loc1* = timeBorders[_parent._currentLevel - 1] / 50;

bonus = Math.floor((timeBorders[_parent._currentLevel - 1] * 2 - getCurrentTime()) / _loc1_ * 20);

}

else

{

bonus = 0;

}

}