EC2 virtual machines are now billed by the second. Previously they were billed by the hour. This is limited to Linux VMs that don't have a marketplace surcharge, but doesn't apply to Windows VMs. I presume this is due to Microsoft licensing.
This will save a bit of money for people who want to experiment, who are using auto scaling, or who have large dynamic fleets of servers. For people with steady state workloads it won't make any difference.
Google Cloud bills by the minute, which is almost as good. I think Azure bills by the hour.