A comprehensive date handling 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.
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.
cl-dates has the following features:
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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.
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Dates can be converted to a string in several formats including ISO 8601 and RFC 822 using the function date->string.
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Functions are provided to calculate astronomical dates for any given year (easter-day, vernal-equinox, autumnal-equinox, summer-solstice, and winter-solstice).
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Holiday calendars: Functions to automatically generate holidays for certain currencies / trading centres are provided. Dates can be checked against these calendars to see if they are business days or not.
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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.
- Other functions (e.g., add-workdays, prev-workday, diff-workdays) provide similar functionality using holiday calendars.
- A function generate-schedule is provided to generate a schedule of dates (e.g., for a bond, loan, credit default swap). This uses date roll conventions and holiday calendars to determine each payment date.
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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.
All functions are provided in the CL-DATES package. A nickname "DT" is defined to reduce typing if the symbols are not imported.
The only dependency is on the Common Lisp package. There are no third party library dependencies.
Test cases are in test-*.lisp. To run, load the cl-dates-test package and run cl-dates-test:run-all-tests.
The library has been tested with Clozure CL (on Mac OS X and Windows) and SBCL (on Mac OS X only). The code is portable and should run on any Common Lisp environment.
cl-dates is licensed under the 2-clause BSD license.
Detailed documentation is yet to be written