Time in Lisp

(defpackage :time-example
  (:use :cl :local-time))

(in-package :time-example)

(defun main ()
  (let ((p #'print))
    ;; We'll start by getting the current time.
    (let ((now (now)))
      (funcall p now)

      ;; You can build a timestamp by providing the
      ;; year, month, day, etc. Times are always associated
      ;; with a timezone.
      (let ((then (encode-timestamp 651387237 58 34 20 17 11 2009 :timezone +utc-zone+)))
        (funcall p then)

        ;; You can extract the various components of the time
        ;; value as expected.
        (funcall p (timestamp-year then))
        (funcall p (timestamp-month then))
        (funcall p (timestamp-day then))
        (funcall p (timestamp-hour then))
        (funcall p (timestamp-minute then))
        (funcall p (timestamp-second then))
        (funcall p (timestamp-microsecond then))
        (funcall p (timestamp-timezone then))

        ;; The day of the week is also available.
        (funcall p (timestamp-day-of-week then))

        ;; These functions compare two times, testing if the
        ;; first occurs before, after, or at the same time
        ;; as the second, respectively.
        (funcall p (timestamp< then now))
        (funcall p (timestamp> then now))
        (funcall p (timestamp= then now))

        ;; The time-difference function returns a duration representing
        ;; the interval between two times.
        (let ((diff (timestamp-difference now then)))
          (funcall p diff)

          ;; We can compute the length of the duration in
          ;; various units.
          (funcall p (duration-as diff :hour))
          (funcall p (duration-as diff :minute))
          (funcall p (duration-as diff :sec))
          (funcall p (duration-as diff :nsec))

          ;; You can use timestamp+ to advance a time by a given
          ;; duration, or with a negative duration to move backwards.
          (funcall p (timestamp+ then diff))
          (funcall p (timestamp+ then (duration- diff))))))))

(main)

This Lisp code provides similar functionality to the original Go example, using the local-time library which is commonly used for time operations in Common Lisp.

Here’s a brief explanation of the changes:

We define a package time-example and use the local-time library.
The main function encapsulates all the operations.
We use now to get the current time, similar to time.Now() in Go.
encode-timestamp is used to create a specific timestamp, similar to time.Date in Go.
Various functions like timestamp-year, timestamp-month, etc., are used to extract components of the timestamp.
timestamp<, timestamp>, and timestamp= are used for time comparisons.
timestamp-difference is used to calculate the duration between two times.
duration-as is used to convert the duration to different units.
timestamp+ is used to add or subtract durations from a timestamp.

Note that Lisp doesn’t have a direct equivalent to Go’s nanosecond precision, so we use microseconds instead. Also, the day of the week is represented as a number (0-6) rather than a named constant.

To run this program, you would need to have a Common Lisp implementation installed along with the local-time library. You can then load and run this file in your Lisp environment.

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