Orders of magnitude (time)

An order of magnitude of time is usually a decimal prefix or decimal order-of-magnitude quantity together with a base unit of time, like a microsecond or a million years. In some cases, the order of magnitude may be implied (usually 1), like a "second" or "year." In other cases, the quantity name implies the base unit, like "century." In most cases, the base unit is seconds or years.

Prefixes are not usually used with a base unit of years. Therefore, it is said "a million years" instead of "a megayear." Clock time and calendar time have duodecimal or sexagesimal orders of magnitude rather than decimal, e.g., a year is 12 months, and a minute is 60 seconds.

The smallest measurable increment of time is Planck time –the time light takes to traverse the Planck distance, many decimal orders of magnitude smaller than a second. However, the Planck time is not a quantum of time, as both general relativity and quantum mechanics treat time as continuous.

The largest realized amount of time, based on known scientific data, is the age of the universe, about 13.8 billion years — the time since the Big Bang as measured in the cosmic microwave background rest frame. Those amounts of time together span 60 decimal orders of magnitude. Metric prefixes are defined spanning 10−30 to 1030, 60 decimal orders of magnitude which may be used in conjunction with the metric base unit of second.

Metric units of time larger than the second are most commonly seen only in a few scientific contexts such as observational astronomy and materials science, although this depends on the author.