Adding my monthly usage billing to my website's dashboard

Hello,

I have a dashboard for a website I run, and I wanted to add my monthly usage bill that it’s currently set at. (For example, for 10 hours @ $0.03/hr, it should show $0.30 on my dashboard.)

The thing is, it’s slightly inaccurate, missing 6 cents. (Essentially, 2 hours is missing.)

I am using PHP and Carbon to do this:

$startOfMonth = new Carbon('first day of this month midnight', 'America/New_York');
$currentDay = new Carbon('now', 'America/New_York');

return $currentDay->diffInHours($startOfMonth);

As of right now, it should be 640+ hours for the duration of this month. I set the timezone as New York (I’m in the UK) as I assumed the billing process is handled at DO’s headquarters which is in NYC.

I am also using Graham Campbell’s Laravel-DigitalOcean repo and it does output an object which tells me the hourly cost.

This is how I’m doing the mathematics:

$droplet->getPriceHourly() * $droplet->getHourDurationThisMonth()

What am I doing wrong to receive this inaccurate output?