This paper presents a new formalism for reasoning about change over
time. The formalism derives a clean separation between the notion of
states and situations. It allows more flexible temporal causal
relationships than do other formalisms for reasoning about causal
change, such as the situation calculus and the event calculus. It
includes effects that start during, immediately after, or some time
after their causes, and which end before, simultaneously with, or
after their causes. A formal distinction between actions, action-types
and events is proposed, which allows the expression of common-sense
causal laws at high level. It is shown how these laws can be used to
deduce state change over time at low level, when events occur under
certain preconditions hold. Two problems that beset most
interval-based temporal systems, i.e., the so-called dividing instant
problem and intermingling problem, are absent from the formalism.