JDToJulian
If you need an easy way to convert a decimal julian day to an unix timestamp you can use:
$unixTimeStamp = ($julianDay - 2440587.5) * 86400;
2440587.5 is the julian day at 1/1/1970 0:00 UTC
86400 is the number of seconds in a day
jdtounix
(PHP 4, PHP 5)
jdtounix — Convert Julian Day to Unix timestamp
Description
int jdtounix
( int
$jday
)
This function will return a Unix timestamp corresponding to the
Julian Day given in jday or FALSE if
jday is not inside the Unix epoch
(Gregorian years between 1970 and 2037 or 2440588 <=
jday <= 2465342 ). The time returned is
localtime (and not GMT).
Parameters
-
jday -
A julian day number between 2440588 and 2465342.
Return Values
The unix timestamp for the start of the given julian day.
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User Contributed Notes
jdtounix - [6 notes]
fabio at llgp dot org ¶
6 years ago
Anonymous ¶
8 years ago
Warning: the calender functions involving julian day operations seem to ignore the decimal part of the julian day count.
This means that the returned date is wrong 50% of the time, since a julian day starts at decimal .5 . Take care!!
pipian at pipian dot com ¶
9 years ago
Remember that UNIX timestamps indicate a number of seconds from midnight of January 1, 1970 on the Gregorian calendar, not the Julian Calendar.
hrabi at linuxwaves dot com ¶
6 years ago
Beware, jd here is not (astronomical or geocentric) Julian Day (JD), but Chronological Julian Day (CJD)! When JD start at noon of UTC time (12:00 UTC), CJD start at midnight at *local* time! Or considering head "Chronlogical Julian Day/Date" at "http://www.decimaltime.hynes.net/dates.html", when day localy start (it should be at sunset for instance).
try this...
<?php
define("UJD", 2440587.5);
define("SEC4DAY", 86400);
function u2j($tm) {
return $tm / SEC4DAY + UJD;
}
function mmd($txt, $str_time) {
$t = strtotime($str_time);
$j = unixtojd($t);
$j_fabio = u2j($t);
$s = strftime('%D %T %Z', $t);
printf("${txt} => %s, CJD: %s, JD: %s<br>\n", $s, $j, $j_fabio);
}
$xt = strtotime("1.1.1970 0:00.00 GMT");
$slb = "-1 day 23:30"; // let CJD be N
$sla = "0:30.00"; // should be N+1
$slm = "0:00"; // should be N+1
$sgmt = "0:00.00 GMT"; // don't forget to observe JD.
mmd("local before", $slb);
mmd("local after", $sla);
mmd("local midnight", $slm);
mmd("GMT midnight", $sgmt);
?>
I got this (you see, JD havn't change day, because UTC noon is far away):
local before => 03/28/07 23:30:00 CEST, CJD: 2454188, JD: 2454188.39583
local after => 03/29/07 00:30:00 CEST, CJD: 2454189, JD: 2454188.4375
local midnight => 03/29/07 00:00:00 CEST, CJD: 2454189, JD: 2454188.41667
GMT midnight => 03/29/07 02:00:00 CEST, CJD: 2454189, JD: 2454188.5
erelsgl dot NOSPAM at cs dot technion dot ac dot il ¶
6 years ago
Just to clarify the differences between the different methods to convert a date to a timestamp.
Suppose:
<?php
$x = JDToUnix(GregorianToJD(9,23,2006));
$y = strtotime('2006-09-23');
$z = (GregorianToJD(9,23,2006) - 2440587.5) * 86400;
?>
Then, on a machine whoze timezone is GMT-0400, we get the following results:
<?php
$x === 1158969600;
$y === 1158984000; // $x + 4 hours
$z === 1159012800; // $x + 12 hours
?>
seb at carbonauts dot com ¶
9 years ago
Remember that unixtojd() assumes your timestamp is in GMT, but jdtounix() returns a timestamp in localtime.
This fooled me a few times.
So if you have:
$timestamp1 = time();
$timestamp2 = jdtounix(unixtojd($timestamp1));
Unless your localtime is the same as GMT, $timestamp1 will not equal $timestamp2.