jdtounix

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

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!!

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.

unixtojd() assumes that your timestamp is in GMT, but jdtounix() returns a timestamp in localtime.
so
<?php
$d1=jdtogregorian(unixtojd(time()));
$d2= gmdate("m/d/Y");
$d3=date("m/d/Y");
?>
$d1 always equals $d2 but $d1 may differ from $d3

Remember that UNIX timestamps indicate a number of seconds from midnight of January 1, 1970 on the Gregorian calendar, not the Julian Calendar.