Calculates a progress value based on time.
It uses the
duration time value its initialised with to calculate a progress value.
The calculator records a start time when we call the
progress method for first time and will produce a progress value of 0 at this point.
The progress value will be 1 when the current time equals the start time the calculator recorded + the duration.
Constructs a time calculator with a duration.
The time in seconds from the start time when the progress value equals 1.
A progress calculator based on time.