This issue only existed on the initial iteration. When timing is paused and stepped,
initialize() and execute() get called with the same timestamp the first time, which
would result in a divide by zero. All subsequent steps advance timing and only
call execute() so the time deltas are all set correctly.
The current hasPeriodPassed() function is confusing. In preparation for deprecating it,
add new advanceIfElapsed() function with same functionality and hasElapsed() function
which only checks that the time period has elapsed and does not advance the timer.
Also fix a couple of incorrect usages of hasPeriodPassed().
