(PHP 4, PHP 5, PHP 7, PHP 8)

unixtojdConvert Unix timestamp to Julian Day


unixtojd(?int $timestamp = null): int|false

Return the Julian Day for a Unix timestamp (seconds since 1.1.1970), or for the current day if no timestamp is given. Either way, the time is regarded as local time (not UTC).



A unix timestamp to convert.

Return Values

A julian day number as integer, or false on failure.


Version Description
8.0.0 timestamp is nullable now.

See Also

  • jdtounix() - Convert Julian Day to Unix timestamp

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User Contributed Notes 6 notes

fabio at llgp dot org
17 years ago
If you need an easy way to convert an unix timestamp to a decimal julian day you can use:

$julianDay = $unixTimeStamp / 86400 + 2440587.5;

86400 is the number of seconds in a day;
2440587.5 is the julian day at 1/1/1970 0:00 UTC.
17 years ago
Its clearly stated that this function returns the Julian Day, not Julian Day + time.

If you want the time with it you will have to do something like:

unixtojd at isslow dot com
4 months ago
unixtojd is slow.
Direct arithmetics calculations are faster and still coherent with original unixtojd.

Feel free to add a test on $timestamp to set it to time() when $timestamp is null.

function fast_unixtojd($timestamp){
return intval($timestamp / 86400 + 2440588);

$time = time();
$t_unixtojd = 0;
$t_fast_unixtojd = 0;
for ($t = $time - 240 * 3600; $t < $time; $t++) {
$time1 = microtime(true);
$a = unixtojd($t);
$time2 = microtime(true);
$b = fast_unixtojd($t);
$time3 = microtime(true);
if ($a != $b) {
echo "$a $b $t\n";
$t_unixtojd += $time2 - $time1;
$t_fast_unixtojd += $time3 - $time2;
echo "unixtojd: $t_unixtojd sec\nfast_unixtojd: $t_fast_unixtojd sec\n";

unixtojd: 0.42854166030884 sec
fast_unixtojd: 0.13218021392822 sec
hrabi at linuxwaves dot com
17 years ago
according to http://www.decimaltime.hynes.net/dates.html#jd and reading "X. Calendar Functions" on this side, it seems that php "jd" is precisely mean as "Chronological Julian Day" (should it be named cjd, and primarily strictly mentioned - isn't it?), used for covnersion between calendar systems. Than it's ok (but Incomplete manual is strongly confusing here IMHO).
Even that, cJD is adjusted to a local time, so... I am rather babeled now, so nothing else :-).
hrabi at linuxwaves dot com
17 years ago
This is unusable. Julian Day start at noon, not midnight. It's better to use Fabio solution (however there is a lurk problem with leap second).

function mmd($txt, $str_time) {
$t = strtotime($str_time);
$j = unixtojd($t);
$s = gmstrftime('%D %T %Z', $t);
$j_fabio = $t / 86400 + 2440587.5;

printf("${txt} => (%s) %s, %s U, %s J, or %s J<br>\n", $str_time, $s, $t, $j, $j_fabio);

//$xt = strtotime("1.1.1970 15:00.00 GMT");
$sam = "9.10.1995 02:00.01 GMT";
$spm = "9.10.1995 22:00.01 GMT";

// unixtojd for $spm returns 2450000 (OK), but for $sam returns 2450000 too! (it is wrong).
mmd("am", $sam); // should be 2449999 (+ 0.58334)
mmd("pm", $spm); // should be 2450000 (+ 0.41668)

unix time, and UTC, TAI, ntp, ... problems: http://en.wikipedia.org/wiki/Unix_time
Julian Date Converter: http://aa.usno.navy.mil/data/docs/JulianDate.html
history overview: http://parris.josh.com.au/humour/work/17Nov1858.shtml
johnston at capsaicin dot ca
20 years ago
Also note that epoch is in UTC time (epoch is a specific point in time - epoch is not different for every time zone), so be aware of timezone complexities.
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