The Julian Date (JD) is the time on the Greenwich meridian measured as the number of days since noon on January 1st 4,713 BC. It is useful in providing a uniform scale of days for recording astronomical events and observations without the complications of years, months and days in various calendars. It makes plotting a graph of the brightness variations of a star over a longish period easier, for example. Starting at noon is important to remember. For European and African observers it means there is no change of date during a night.
As you can see on these pages, the value of JD is now quite large. So it is common practice to use a modified JD in which, say, the first 3 digits are chopped off. Data should include a statement when that is being done.
Java (like several other programming languages) keeps track of time by counting milliseconds from midnight at the beginning of 1970 UT. So there is a constant offset between Java time and Julian Date, making it easy to display as above. Note that this display is only as accurate as the clock in your computer. UT is Universal Time which, for amateur purposes, is identical to Greenwich Mean Time, so it is the time at Greenwich. Observers in other time zones have to add an offset to get their local time, which their computer system will do automatically. Julian Date does not involve any time zones.
A calendar on the page about Jupiter's satellites may be used for converting between Gregorian calendar dates and Julian dates.