So I added this to the Wikipedia definition:

> Note: There is a further technical condition that \\(\phi^t\\) is an action of T on M. That includes the facts that \\(\phi^0\\) is the identity function and that \\(\phi^{s+t}\\) is the composition of \\(\phi^s\\) and \\(\phi^t\\). This is a semigroup action, which doesn't require the existence of negative values for t, and doesn't require the functions \\(\phi^t\\) to be invertible.