How to convert Unix timestamp to Datetime in Common Lisp

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    (defun unix-timestamp-to-date (unix-timestamp)
    (let* ((epoch (encode-universal-time 0 0 0 1 1 1970))
            (time (float unix-timestamp)))
        (multiple-value-bind (second minute hour day month year)
            (decode-universal-time (+ (coerce time 'integer) epoch))
        (format nil "~4d-~2,'0d-~2,'0d ~2,'0d:~2,'0d:~2,'0d" year month day hour minute second))))
    
    ;; Convert Unix timestamp 1093730212 to date
    (format t "Converted Date: ~a~%" (unix-timestamp-to-date 1093730212))

Output:

Converted Date: 2004-08-28 21:56:52

Example only. There may be multiple ways to perform this operation.

Code Explanation

In this Common Lisp example the encode-universal-time function creates a time object representing the Unix epoch (1970-01-01 00:00:00 UTC).
The Unix timestamp is then added to this epoch. The decode-universal-time function decodes the combined timestamp into its individual time components (year, month, day, hour, minute, second).
Finally, format is used to print the result in a human-readable format, such as '2004-08-28 21:56:52'.

Convert Date to Unix timestamp in Common Lisp

Other useful Common Lisp date functions

Function	Description	Example
`encode-universal-time`	Encodes date and time information into a universal time (seconds since the Common Lisp epoch).	`(encode-universal-time 45 30 8 15 5 2022)` returns the encoded time for 2022-05-15 08:10:45.
`decode-universal-time`	Decodes a universal time (in seconds) into a human-readable date (year, month, day, hour, minute, second).	`(decode-universal-time (get-universal-time))` returns the current date and time.
`get-universal-time`	Returns the current universal time, the number of seconds since the Common Lisp epoch (January 1, 1900).	`(get-universal-time)` returns the current timestamp.
`get-decoded-time`	Returns the decoded time for the current universal time, providing components like year, month, day, etc.	`(get-decoded-time)` returns the current date and time as multiple values.
`multiple-value-bind`	Allows extracting multiple values returned by functions such as `decode-universal-time`.	`(multiple-value-bind (sec min hr day mon yr) (decode-universal-time (get-universal-time)) ...)`

Current Unix Timestamp SECONDS SINCE JAN 01 1970

Current UTC Time

2025-08-20