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

// (algorithm adopted from Press et al.)

//-------

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;

        }

    }

}