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TimePeriodsCowan

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== Time Durations and Intervals ==

A ''duration'' is an immutable object belonging to a distinct 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 the 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.

== Duration procedures ==

`(make-duration `''chronology''` `''alist''`)`

Constructs and returns a duration based on ''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`, `seconds`, and `ms' (milliseconds).
The values of each field must be exact integers,
except that the smallest unit present may be an exact rational number.

`(duration? `''obj''`)`

Returns `#t` if ''obj'' is a duration, and `#f` otherwise.

`(duration-`''unit''` `''duration''`)`

Returns the value associated with ''unit''.  For example, `(duration-weeks d)` returns the number of weeks in duration ''d''.  If the duration was not constructed with a
particular unit, `#f` is returned instead.


== Time interval procedures ==

TODO

time

2010-11-25 02:28:15