When I start the timer and call getLastActiveTime function, getLastActiveTime returns false.
$( document ).idleTimer();
var time = $( document ).idleTimer("getLastActiveTime");
Tracing through the code, it looks like firstParam is undefined. So line 250 skipped over because undefined === null is false, and ends up returning false at line 255.
My current workaround is to call idleTimer(null). In which case, lastActiveTime correctly returns the last active time.
$( document ).idleTimer(null);
var time = $( document ).idleTimer("getLastActiveTime");