A duration is an immutable object of a disjoint type and representing a period of time according a particular chronology. In the ISO, Gregorian, and Julian chronologies, it is the sum of some number of years, months, weeks, days, hours, minutes, and seconds.
A time interval is an immutable object belonging to a distinct type and representing the elapsed time between two instants of time. See TimeAdvancedCowan for the definition of instants and date objects.
(make-duration [ chronology ] alist)
Returns a duration based on chronology, defaulting to the value of (current-chronology). Alist is an alist mapping duration field names understood by that chronology to their values. In the ISO, Gregorian, and Julian calendars, the field names are years, months, weeks, days, hours, minutes, and seconds. The values of each field must be exact integers.
Returns #t if obj is a duration, and #f otherwise.
Returns a newly allocated alist mapping field names existing in duration into their values. Fields mapped to #f don't appear in the alist.
(duration-field fieldname duration)
Returns the value associated with fieldname. For example, (duration-field 'weeks d) returns the number of weeks in duration d. If the duration was not constructed with a particular unit, #f is returned instead. No conversion is done: a 7-day interval is not considered equivalent to a 1-week interval, for example.
An interval represents the time between two or more instants, possibly repeated more than once. Intervals belong to an immutable disjoint type.
(make-interval date-or-duration-1 date-or-duration-2 [ repetition ] )
Returns an interval which:
An error is signaled if both arguments are durations, or if the arguments do not share the same chronology, or the dates do not have enough fields to specify an instant.
The repetition argument specifies how many times the interval is repeated, 1 by default. If the argument is #t, the interval is repeated indefinitely.
Returns #t if obj is an interval and #f otherwise.
Returns the start date, stop date, duration, or repetition count of interval. Depending on how the interval was constructed, at least one of these will have to be calculated.