Records the time since the input changed to false
Timer that starts at the initial time with a value of 0,
and gets reset each time the input signal switches to false.
For example, if the simulation starts at t = 10 and at t=11,
the input becomes false, then the timer outputs
y=t-10 for t < 11, and y=t-11 afterwards, unless
the input becomes false again.
- 
May 12, 2017, by Michael Wetter:
 Corrected implementation for non-zero start time.
 This is for
issue 780.
- 
January 4, 2011, by Wangda Zuo and Michael Wetter:
 Revised implementation.
- 
February 12, 2009, by Michael Wetter:
 First implementation.
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