There is no equation to calculate time because it is not a physical quantity. Time is a dimension that measures the flow of events. You can compare events on Earth, for instance, as measured by our clocks which depend on the rotational and revolution periods of the planet with events of much greater energy like in subatomic particle decay experiments involving particles that live for 10^(-27) seconds measured by clocks based on cesium 133 atoms vibrating about 9 billion times per second. But you can calculate durations between events.