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Converting DMYHMS to Seconds

Problem

You want to convert a date, a time, or both with distinct values for day, month, year, etc. to seconds.

Solution

First, it is important to note that time (in PLT Scheme) is always an exact integer based on a platform-specific starting date, an Epoch, (with a platform-specific minimum and maximum value). Having that into consideration, and using Mzlib, we can get the number of seconds of a given date, and time using find-seconds. For instance, to get the seconds corresponding to the date, and time: "Wed Apr 21 13:01:01-0500 2004" you could simply:

(require (lib "date.ss"))

(let ((seconds 1)
      (minutes 1)
      (hour 13)
      (day 21)
      (month 4)
      (year 2004))
  (find-seconds seconds minutes hour day month year))

which will return 1082570461, and that is the number we were looking for. You can use the recipe in TimeToday to get the date, time, and other information from that number of seconds.

Discussion

In the previous recipe, TimeToday, we converted a number of seconds to an instance of the date datatype. It would be helpful to have procedures that acted on date instances to get back a corresponding number of seconds. The library Mzlib provides the procedure date->julian/scalinger which converts a date structure (up to 2099 BCE Gregorian) into a Julian date number.

The returned value is not a strict Julian number, but rather Scalinger's version, which is off by one for easier calculations. The Mzlib library also provides the procedure julian/scalinger->string to convert a Julian number into a string. Here's an example:

(require (lib "date.ss"))

(let* ((seconds (current-seconds))
       (today (seconds->date seconds))
       (julian (date->julian/scalinger today)))
  (values julian
          (julian/scalinger->string julian)))

which would return values like these: 2453122, and "JD 2 453 122".

SRFI 19, as mentioned before, is a very complete time library, and it includes a comprehensive list of procedures to convert date structures to different time systems. Here's a list of the procedures supported:

date->julian-day

Converts date to Julian Day.

date->modified-julian-day

Converts date to Modified Julian Day.

date->time-monotonic

Converts date to monotonic time.

date->time-tai

Converts date to TAI (International Atomic Time) time.

date->time-utc

Converts date to UTC time.

And here's a very simple example:

(require (lib "19.ss" "srfi"))

(let ((today (current-date)))
  (values (date->julian-day today)
          (date->modified-julian-day today)
          (date->time-monotonic today)
          (date->time-tai today)
          (date->time-utc today)))

note that some of these return an instance of another (internal) structure, time, which can be manipulated with other procedures in SRFI 19, as those seen in recipe TimeEpochToTime.

Comments here

-- FranciscoSolsona - 28 Apr 2004