How does hourly charging work for fractional hours?

If I start a droplet, let it run for 15 minutes, then destroy it, then start it again and run for another 15 minutes, will I be charged for one hour (30 minutes total use, rounded to the next hour), or is each 15 minute use rounded to the nearest hour? Does it matter if the two sessions were within the same 60 minute period? What if I run a droplet 15 minutes each day, then destroy it. Is each 15 minute session rounded to an hour or are they summed and rounded to the next whole hour?

I understand that if I run two droplets simultaneously for 15 minutes each, that will be rounded to 2 hours.