The formula is 100% minus 100% times downtime/time in month/year.
153 is the number of minutes they were down going off the reported updates at https://status.aws.amazon.com/ - 11:35AM PST was when they fixed the status page, 2:08PM PST was when S3 was fully back online. (And 153 is underestimating it, because there were errors going on for long before they fixed the status page, but I don't have timestamps on that.)
153 is the number of minutes they were down going off the reported updates at https://status.aws.amazon.com/ - 11:35AM PST was when they fixed the status page, 2:08PM PST was when S3 was fully back online. (And 153 is underestimating it, because there were errors going on for long before they fixed the status page, but I don't have timestamps on that.)