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"YYYY-MM-DD" to Time in Common Lisp

muro on March 04, 2024

Disclaimer: The purpose of this post is to track my journey into Common Lisp. Though there might exist more optimal ways, this represents my pers...
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7stud profile image
7stud • • Edited

The function encode-universal-time doesn't give me the same results as you. Your results (shown in the comments):

(encode-universal-time 0 0 0 21 1 23) ;; returns 3883233600
(encode-universal-time 0 0 0 22 1 23) ;; returns 3883320000
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My results (with SBCL 2.4.0):

CL-USER> (encode-universal-time 0 0 0 21 1 23)
3883273200
CL-USER> (encode-universal-time 0 0 0 22 1 23)
3883359600
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And, I get yet a third result when I use an online lisp compiler:

(format t "~d~%" (encode-universal-time 0 0 0 21 1 2023))  ;; => 3883248000
(format t "~d~%" (encode-universal-time 0 0 0 22 1 2023))  ;; => 3883334400
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I'm not sure why, but changing the year from 23 to 2023 produces the same results:

CL-USER> (encode-universal-time 0 0 0 21 1 2023)
3883273200
CL-USER> (encode-universal-time 0 0 0 22 1 2023)
3883359600
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There's nothing in the hyperspec about that. How does encode-universal-time know that 23 means 2023 and not 1923? Edit: The hyperspec says:

A decoded time is an ordered series of nine values that, taken together, represent a point in calendar time (ignoring leap seconds)
...

...

Year
An integer indicating the year A.D. However, if this integer is between 0 and 99, the "obvious" year is used; more precisely, that year is assumed that is equal to the integer modulo 100 and within fifty years of the current year (inclusive backwards and exclusive forwards). Thus, in the year 1978, year 28 is 1928 but year 27 is 2027. (Functions that return time in this format always return a full year number.)

Edit: I think the different results have to do with timezones. According to the hyperspec for encode-universal-time:

If time-zone is supplied, no adjustment for daylight savings time is performed.

That implies that there is some adjustment if you don't supply a timezone? I tried suppling a timezone of 0, which I think is GMT:

CL-USER> (encode-universal-time 0 0 0 21 1 2023 0)
3883248000

CL-USER> (encode-universal-time 0 0 0 22 1 2023 0)
3883334400
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With a timezone of 0, I got the same numbers using the online lisp interpreter.

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vindarel profile image
vindarel •

Oh, on reddit we have the idea:

(parse-integer "2023-01-21" :end 4)
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mrmuro profile image
muro •

Thanks @vindarel !, should that simplify my parse-date function? something like:

(defun parse-date (date-string)
  "Extracts year, month, and day from a date string formatted as YYYY-MM-DD."
  (let ((year (parse-integer date-string :start 0 :end 4))
        (month (parse-integer date-string :start 5 :end 7))
        (day (parse-integer date-string :start 8 :end 10)))
    (values year month day)))
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vindarel profile image
vindarel • • Edited

Well done. To add a handy library in:

(local-time:parse-timestring "2042-11-13")
;; => @2042-11-13T01:00:00.000000+01:00
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