It is a convention, often used to differentiate blocking and asynchronous methods in the APIs (e.g. `Read` and `ReadAsync`, etc). You're not required to use it, but it is useful whenever there is a chance of confusion.
As for the timeouts, it would be strange to bake this into a language (different platforms may support different timers, at the very least).
Instead, you use library for this[1]:
int timeout = 1000;
var task = SomeOperationAsync();
if (await Task.WhenAny(task, Task.Delay(timeout)) == task) {
// task completed within timeout
} else {
// timeout logic
}