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 # cl-dates
-Date-time library for Common Lisp
 
-This library contains a fairly comprehensive set of functions to handle dates with somewhat weaker support for date-times. It is useful for applications where dates are an important data type e.g., business or financial calculations.
+*A comprehensive date handling library for Common Lisp*
 
-Features provided are:
-- Conversion from a string to date: a wide variety of date time formats are accepted (see test-parse-date.lisp for examples)
-- Printing dates in various formats
-- Computation of special dates / datetimes - Easter, Spring / Autumn equinoxes, Summer / Winter Solstice
-- Date arithmetic including support for business dates (holiday processing)
-- Date comparisons
-- Miscellaneous date functions e.g., computing the 3rd Wednesday of a month
+This library contains a fairly comprehensive set of functions to
+handle dates with somewhat weaker support for date-times. It is useful
+for applications where dates are an important data type e.g., business
+or financial calculations.
 
-The library does not have any dependencies on third party libraries and is provided under a BSD licence.
+Dates are stored as a floating point number whose value is the Julian
+day number plus the day fraction corresponding to the time. Since a
+Julian date, by definition starts at noon, the date is an integer only
+when the time is exactly noon. For example, "January 1, 2017" can have
+values from 2457754.5 to 2457755.499*. As a result, the date
+arithmetic functions provided work off the Julian day number (2457755
+in this case) and ignore the time.
 
-*The version currently uploaded is missing all the business date functions but is otherwise usable.*
+## Features
+
+*cl-dates* has the following features:
+
+- *Parse a date from a string*: Largely based on the Python
+  *dateutils* library, a complex parser for commonly encountered date
+  formats is provided in the function **string->date**. The test cases
+  in *test-parse-date.lisp* provide a flavour of what kinds of strings
+  are acceptable.
+
+- Dates can be *converted to a string* in several formats including
+  ISO 8601 and RFC 822 using the function **string->date**.
+
+- Functions are provided to calculate *astronomical dates* for any
+  given year (**easter-day**, **vernal-equinox**,
+  **autumnal-equinox**, **summer-solstice**, and **winter-solstice**).
+
+- *Date arithmetic*
+    - Functions are provided to add or subtract days or months from a
+      date (**date+**, **date-**, **add-months**,
+      **add-years**). Addition of months or years correctly handles
+      cases where the given date falls on a month end.
+    - Compute differences between dates (**diff-days**,
+      **diff-years**). When computing the number of years, the day
+      convention (such as *Actual/365* or *30/360*) is taken into
+      account so that the result can be used to accurately compute
+      interest on a bond or loan.
+
+- *Miscellaneous* date manipulation functions are provided to compute
+  the last day of a month, last day of the previous month, Nth day of
+  week in a month (e.g., 3rd Wednesday), etc.
+
+## Dependencies
+
+The only dependency is on the Common Lisp package. There are no third
+party library dependencies.
+
+## License
+
+*cl-dates* is licensed under the 2-clause BSD license.
+
+## To Do
+
+Date arithmetic functions that use holidays and weekends for
+adjustments are not yet included.