For time-valued quantities, in addition to the definite duration UCUM units, CQL defines calendar duration keywords to support calendar-based durations and arithmetic. For example, UCUM defines an annum (`'a'`) as 365.25 days, whereas the year (`'year'`) duration in CQL is specifically a calendar year. This difference is important, especially when performing calendar arithmetic. // Example of using a calendar-based calculation versusu a definite-time duration calculation See the definition of the [Quantity] ( https://cql.hl7.org/2020May/02-authorsguide.html#quantities ) type in the CQL Author's Guide, as well as the [Date/Time Arithmetic] ( https://cql.hl7.org/2020May/02-authorsguide.html#datetime-arithmetic ) discussion for more information.