getTime() not consistently reporting correct time information.

I've tried creating an electric pulse meter with the puck js 2.1 The code works on occasion but then starts reporting funny values for watts. The general idea of the code is to get the current time, compare it with the previous time of the last pulse and use the time difference to calculate the watts. Sometimes however, even though i can tell from the logs that say the last pulse time was 12 or so seconds ago, the calculation only reads .01 of a second therefore throwing the calculation off. It's not consistent, sometimes it gets it bang on, other times not. For instance I ran it yesterday for the first time, it seem to run fine for a few hours but then started reporting erroneous values. When i ran updated code 10 minutes ago with better logging, i can see that every second reading is correct. Note originally i was using Date.now() for the time, but then tried getTime() to see if it would fix the issue, but it did not. Please see logs and code below.

var watthours = 0;
var lastpulse = getTime();
var watts = 0;
var battery = Puck.getBatteryPercentage();
var debug = 1;

function advertise() {
	NRF.setAdvertising([
		0x0e, 0xff, 0xa0, 0x83, 
		watthours, watthours >> 8, watthours >> 16, watthours >> 24,
		watts, watts >> 8, // current usage
		battery, // battery %
		0x00, 0x00, 0x00, 0x00
	], {
		name: "The Q Meter 3000",
		interval: 600
	});
}

function calculate() {
	var now = getTime();
	var pulsetime = now - lastpulse;

	if (debug) {
		console.log(`\n${now} now`);
		console.log(`${pulsetime} pulse time`);
		console.log(`${lastpulse} last pulse`);
	}
	lastpulse = now;
	watts = 3600 / pulsetime;
	watts = Number(watts.toFixed(0));
	if (debug) {
		console.log(`${watts} watts`);
		console.log(`${watthours} watt hours`);
		console.log(`${battery}% battery`);
	}
}


// Set up pin states
D1.write(0);
pinMode(D2, "input_pullup");

// Watch for pin changes
setWatch(function(e) {
	watthours++;

	if (watthours % 720 == 0) {
		battery = Puck.getBatteryPercentage();
	}

	calculate();

	if (watthours > 2) {
		advertise();
	}
}, D2, {
	repeat: true,
	edge: "falling"
});

Here are the logs

1666997346.05803203582 now
0.00997924804 pulse time
1666997346.04805278778 last pulse
360748 watts
122 watt hours
76% battery
1666997359.96943950653 now
13.91140747070 pulse time
1666997346.05803203582 last pulse
258 watts
123 watt hours
76% battery
1666997359.97951030731 now
0.01007080078 pulse time
1666997359.96943950653 last pulse
357469 watts
124 watt hours
76% battery
1666997373.88667583465 now
13.90716552734 pulse time
1666997359.97951030731 last pulse
258 watts
125 watt hours
76% battery
1666997373.89662456512 now
0.00994873046 pulse time
1666997373.88667583465 last pulse
361855 watts
126 watt hours
76% battery
1666997387.80678081512 now
13.91015625 pulse time
1666997373.89662456512 last pulse
258 watts
127 watt hours
76% battery
1666997387.81676006317 now
0.00997924804 pulse time
1666997387.80678081512 last pulse
360748 watts
128 watt hours
76% battery