Website Uptime Calculator
In the context of web hosting, uptime is the amount of time the web host server (and your website) is available and operational.
Uptime is an important metric to factor in when choosing a web host and this metric is often stated as a percentage such as "99.9% uptime guarantee".
For example a 99.9% uptime equates to a yearly downtime of 8.76 hours.
Uptime and availability can be used synonymously, as long as the items being discussed are kept consistent. That is, a system can be up, but its services are not available, as in the case of a network outage. This can also be viewed as, a system can be available to work on, but its services are not up from a functional perspective (as opposed to software service/process perspective). The perspective is important here, whether the item being discussed is the server hardware, server OS, functional service, software service/process...etc. Keep the perspective consistent throughout a discussion, then up time and availability can be used synonymously.
Source: Wikipedia, digital.com