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JavaScript JS Julian Day Number Converter - Algorithm Editing

Aevik

New Coder
So..
Here is a happy little bi-directional Julian Day Number / Calendar Date Converter
written in Javascript.

By editing this bit of code (for example) :

Code:
    //round to nearest second
    var jd0 = (intgr + frac)*100000;
    var jd  = Math.floor(jd0);
    if( jd0 - jd > 0.5 ) ++jd;
    return jd/100000;

the Julian Day Number epoch (or output) can either be manipulated, or cause a digital aneurysm, depending on whether the editor is an actual coder, or not.

Also -- managing to edit the algorithm to successfully convert in one direction
does not mean a successful conversion in the opposite direction.

How then can the algorithm be edited to output an integer day number + fractional day
from a new epoch -- JD 2049857.25 (for example)
15 March 900 CE at 18:00:00 UTC in the Julian Calendar,
and output correctly in both directions? (based on this example epoch)

Thanks in advance!

JavaScript:
<SCRIPT LANGUAGE="JavaScript" TYPE="text/javascript">
<!-------
//-------
// convert calendar to Julian date
//-------
function cal_to_jd( era, y, m, d, h, mn, s )
{
     var jy, ja, jm;               //scratch

     if( y == 0 ) {
          alert("There is no year 0 in the Julian system!");
        return "invalid";
    }
    if( y == 1582 && m == 10 && d > 4 && d < 15 ) {
          alert("The dates 5 through 14 October, 1582, do not exist in the Gregorian system!");
        return "invalid";
    }

//     if( y < 0 )  ++y;
    if( era == "BCE" ) y = -y + 1;
     if( m > 2 ) {
          jy = y;
          jm = m + 1;
     } else {
          jy = y - 1;
          jm = m + 13;
     }

     var intgr = Math.floor( Math.floor(365.25*jy) + Math.floor(30.6001*jm) + d + 1720995 );

     //check for switch to Gregorian calendar
    var gregcal = 15 + 31*( 10 + 12*1582 );
     if( d + 31*(m + 12*y) >= gregcal ) {
          ja = Math.floor(0.01*jy);
          intgr += 2 - ja + Math.floor(0.25*ja);
     }

     //correct for half-day offset
     var dayfrac = h/24.0 - 0.5;
     if( dayfrac < 0.0 ) {
          dayfrac += 1.0;
          --intgr;
     }

     //now set the fraction of a day
     var frac = dayfrac + (mn + s/60.0)/60.0/24.0;

    //round to nearest second
    var jd0 = (intgr + frac)*100000;
    var jd  = Math.floor(jd0);
    if( jd0 - jd > 0.5 ) ++jd;
    return jd/100000;
}

//-------
// convert Julian date to calendar date
//-------
function jd_to_cal( jd, form )
{
     var     j1, j2, j3, j4, j5;               //scratch

     //
     // get the date from the Julian day number
     //
    var intgr   = Math.floor(jd);
    var frac    = jd - intgr;
    var gregjd  = 2299161;
     if( intgr >= gregjd ) {                    //Gregorian calendar correction
          var tmp = Math.floor( ( (intgr - 1867216) - 0.25 ) / 36524.25 );
          j1 = intgr + 1 + tmp - Math.floor(0.25*tmp);
     } else
          j1 = intgr;

     //correction for half day offset
     var dayfrac = frac + 0.5;
     if( dayfrac >= 1.0 ) {
          dayfrac -= 1.0;
          ++j1;
     }

     j2 = j1 + 1524;
     j3 = Math.floor( 6680.0 + ( (j2 - 2439870) - 122.1 )/365.25 );
     j4 = Math.floor(j3*365.25);
     j5 = Math.floor( (j2 - j4)/30.6001 );

     var d = Math.floor(j2 - j4 - Math.floor(j5*30.6001));
     var m = Math.floor(j5 - 1);
     if( m > 12 ) m -= 12;
     var y = Math.floor(j3 - 4715);
     if( m > 2 )   --y;
     if( y <= 0 )  --y;

     //
     // get time of day from day fraction
     //
     var hr  = Math.floor(dayfrac * 24.0);
     var mn  = Math.floor((dayfrac*24.0 - hr)*60.0);
           f  = ((dayfrac*24.0 - hr)*60.0 - mn)*60.0;
     var sc  = Math.floor(f);
           f -= sc;
    if( f > 0.5 ) ++sc;

    if( y < 0 ) {
          y = -y;
        form.era[1].checked = true;
    } else
        form.era[0].checked = true;

    form.year.value          = y;
    form.month[m-1].selected = true;
    form.day[d-1].selected   = d;
    form.hour.value          = hr;
    form.minute.value        = mn;
    form.second.value        = sc;
}

//-------
// calculate Julian date from calendar date or calendar date from Julian date
//-------
function JDcalc( form ) {
    var era;
    for( k=0; k < form.era.length; ++k ) {
        if( form.era[k].checked ) {
            era = form.era[k].value;
            break;
        }
    }
    var calctype;
    for( k=0; k < form.calctype.length; ++k ) {
        if( form.calctype[k].checked ) {
            calctype = form.calctype[k].value;
            break;
        }
    }
    if( calctype == "JD" ) {
        var m;
        for( var k=0; k < form.month.length; ++k ) {    //Netscape 4.7 is stoopid
            if( form.month[k].selected ) {
                m = k+1;
                break;
            }
        }
        var d;
        for( var k=1; k <= form.day.length; ++k ) {    //Netscape 4.7 is stoopid
            if( form.day[k-1].selected ) {
                d = k;
                break;
            }
        }
        var y  = parseFloat(form.year.value);
//      var m  = parseFloat(form.month.value);
//      var d  = parseFloat(form.day.value);
        var h  = parseFloat(form.hour.value);
        var mn = parseFloat(form.minute.value);
        var s  = parseFloat(form.second.value);
        if( y < 0 ) {
              y   = -y;
            era = "BCE";
            form.year.value = y;
            form.era[1].checked = true;
        }
        form.JDedit.value = cal_to_jd(era,y,m,d,h,mn,s);
    } else {
        jd_to_cal(form.JDedit.value,form);
    }
    //weekday
    var     weekday = new Array("Monday","Tuesday","Wednesday","Thursday","Friday","Saturday","Sunday");
    var t  = parseFloat(form.JDedit.value) + 0.5;
    var wd = Math.floor( (t/7 - Math.floor(t/7))*7 + 0.000000000317 );   //add 0.01 sec for truncation error correction
    form.wkday.value = weekday[wd];
}

var     month = new Array("January","February","March","April","May","June","July",
                           "August","September","October","November","December");
var numdays = new Array(31,28,31,30,31,30,31,31,30,31,30,31);

function ModifiedDate() {
     updated = new Date(document.lastModified);
     Month   = month[updated.getMonth()];
     Day     = updated.getDate();
     Year    = updated.getYear();
     Hour    = updated.getHours();
     Min     = updated.getMinutes();
    if( Year < 2000 ) Year += 1900;
    if( Year < 2000 ) Year += 100;  //Netscape 3 and IE 4.7 return 0 instead of 100 for 2000
     if( Hour < 10 )  Hour = "0" + Hour;
     if( Min  < 10 )  Min  = "0" + Min;
     document.write("Last modified " + Month + " " + Day +  ", " + Year + " at " + Hour + ":" + Min);
}

function set_form_fields(form) {

    //grab the current USNO clock time
    var today = new Date();

    var year = today.getYear();
    if( year < 2000 ) year += 1900;
    if( year < 2000 ) year += 100;  //Netscape 3 and IE 4.7 return 0 instead of 100 for 2000
    var month = today.getMonth();
    var day   = today.getDate();
    var hour  = today.getHours();
    var mins  = today.getMinutes();
    var secs  = today.getSeconds();

    //convert to UT
    var TZ = today.getTimezoneOffset()/60;
    hour += TZ;
    if( hour >= 24 ) {
        hour -= 24;
        ++day;
        if( day > numdays[month-1] ) {
            day = 1;
            ++month;
            if( month > 11 ) {
                month -= 12;
                ++year;
            }
        }
    }

    //set the form fields
    form.year.value = year;
    for( k=0; k < form.month.length; ++k ) {    //Netscape 4.7 is stoopid
        if( k == month ) {
            form.month[k].selected = true;
            break;
        }
    }
    form.day.value = day;
    for( k=1; k <= form.day.length; ++k ) {    //Netscape 4.7 is stoopid
        if( k == day ) {
            form.day[k-1].selected = true;
            break;
        }
    }
    form.hour.value   = hour;
    form.minute.value = mins;
    form.second.value = secs;

    doJD(form);
}

function dodate(form) {
    for( k=0; k < form.calctype.length; ++k ) {
        if( form.calctype[k].value == "date" ) {
            form.calctype[k].checked = true;
            break;
        }
    }
}

function doJD(form) {
    for( k=0; k < form.calctype.length; ++k ) {
        if( form.calctype[k].value == "JD" ) {
            form.calctype[k].checked = true;
            break;
        }
    }
}
//------>
</script>
 

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