Parse Unix Time to Date/Time

[Wulfie Reanimator]

Random curiosity caught me, so I made this:

list unix2date(integer unix)
{
    integer sec = unix % 60;
    integer min = (unix/60) % 60;
    integer hour = (unix/60/60) % 24;

    // Epoch started on Thursday. 0 is Monday.
    integer weekday = (unix/60/60/24 + 3) % 7;

    integer year = 1970;                       // Epoch started on 1970.
    year += (integer)((float)(unix/60/60/24) / 365.25); // +49 for 2019.

    integer month;
    // http://howardhinnant.github.io/date_algorithms.html#civil_from_days
    // doe/yoe = day/year of era (era = 400 years), doy = day of year
    integer z = unix / 86400 + 719468;
    integer era;

    if (z >= 0) era = z / 146097;
    else        era = z - 146096 / 146097;

    integer doe = (z - era * 146097);
    integer yoe = (doe - doe/1460 + doe/36524 - doe/146096) / 365;
    integer doy =  doe - (365*yoe + yoe/4 - yoe/100);
    integer m   = (5*doy + 2) / 153;

    if (m < 10) m += 3;
    else        m += -9;
    month = m;

    // llOwnerSay(llList2CSV([unix, year, month, weekday, hour, min, sec]));
    // Example: 2019, 8, 4, 0, 46, 13
    return [year, month, weekday, hour, min, sec];
}

Calculation of the month was only translated by me from the Github page. I still couldn't figure out how to get the day of the month, instead of day of the week.

This is how you might use the function:

list days = [
    "Mon",
    "Tue",
    "Wed",
    "Thu",
    "Fri",
    "Sat",
    "Sun"
];

list months = [
    "Error",
    "Jan",
    "Feb",
    "Feb",
    "Mar",
    "Apr",
    "May",
    "Jun",
    "Jul",
    "Aug",
    "Sep",
    "Oct",
    "Nov",
    "Dec"
];

list unix2date(integer unix)
{
    // ...
}

default
{
    state_entry()
    {
        list out = unix2date(llGetUnixTime());

        llOwnerSay(
            "Year " + llList2String(out, 0) +
            ", " + llList2String(months, llList2Integer(out, 1)) +
            ", " + llList2String(days, llList2Integer(out, 2)) +
            ", at " + llList2String(out, 3) +
            ":" + llList2String(out, 4) +
            ":" + llList2String(out, 5)
        );
        // Example: Year 2019, Jul, Fri, at 0:56:14
    }
}

 

Edited August 30, 2019 by Wulfie Reanimator

[Rolig Loon]

Very nice work.  Compare this with Void Singer's solution at  http://wiki.secondlife.com/wiki/Unix2StampLst , which also calculates the day of the month, using a different algorithm.    It's lovely when independent methods lead to the same result.  There's magic in both of your methods.  I'd try to figure out how they work, but it would make my head hurt. 

[Madelaine McMasters]

My first exposure to calendar algorithms was from programming an old HP-67 calculator. There was a little book of calendar programs that I faithfully keyed in, after which I went looking for dates in history to figure out if they fell on the weekend, when people should have been playing/praying.

Here's a link to one of those programs...
https://www.hpmuseum.org/software/41td2/daydte.htm

I think that program is based on Zeller's Congruence...
https://en.wikipedia.org/wiki/Zeller's_congruence

The HP calculator didn't have a "mod" operator, so rounding and more careful constant selection were used instead.

That HP program was one of many little epiphanies I've had along the way to understanding that math really is nature's language.