// Various date and time functions

// Copyright Peter Hayes 1999-2001, Ole Nielsen 2002-2004


// must be updated using leapyear() if year changed
var month_length=new Array(31,28,31,30,31,30,31,31,30,31,30,31);
var dow = ["Sun","Mon","Tue","Wed","Thu","Fri","Sat"];


function leapyear(year) {
  var leap=false;
  if (year % 4 == 0) leap = true;
  if (year % 100 == 0 ) leap = false;
  if (year % 400 == 0) leap = true;
  return leap;
}


function jd0(year,month,day) {
// The Julian date at 0 hours(*) UT at Greenwich
// (*) or actual UT time if day comprises time as fraction
  var y  = year;
  var m = month;
  if (m < 3) {m += 12; y -= 1};
  var a = Math.floor(y/100);
  var b = 2-a+Math.floor(a/4);
  var j = Math.floor(365.25*(y+4716))+Math.floor(30.6001*(m+1))+day+b-1524.5;
  return j;
}	// jd0()


function jdtocd(jd) {
// The calendar date from julian date, see Meeus p. 63
// Returns year, month, day, day of week, hours, minutes, seconds
  var Z=Math.floor(jd+0.5);
  var F=jd+0.5-Z;
  if (Z < 2299161) {
    var A=Z;
  } else {
    var alpha=Math.floor((Z-1867216.25)/36524.25);
    var A=Z+1+alpha-Math.floor(alpha/4);
  }
  var B=A+1524;
  var C=Math.floor((B-122.1)/365.25);
  var D=Math.floor(365.25*C);
  var E=Math.floor((B-D)/30.6001);
  var d=B-D-Math.floor(30.6001*E)+F;
  if (E < 14) {
    var month=E-1;
  } else {
    var month=E-13;
  }
  if ( month>2) {
    var year=C-4716;
  } else {
    var year=C-4715;
  }
  var day=Math.floor(d);
  var h=(d-day)*24;
  var hours=Math.floor(h);
  var m=(h-hours)*60;
  var minutes=Math.floor(m);
  var seconds=Math.round((m-minutes)*60);
  if (seconds >= 60) {
    minutes=minutes+1;
    seconds=seconds-60;
  }
  if (minutes >= 60) {
    hours=hours+1;
    minutes=0;
  }
  var dw=Math.floor(jd+1.5)-7*Math.floor((jd+1.5)/7);
  return new Array(year,month,day,dw,hours,minutes,seconds);  
}	// jdtocd()


function g_sidereal(year,month,day) {
// sidereal time in hours for Greenwich
  var T=(jd0(year,month,day)-2451545.0)/36525;
  var res=100.46061837+T*(36000.770053608+T*(0.000387933-T/38710000.0));
  return rev(res)/15.0;
}