double x = 12803.000000;
String s = String.format("%06d", (int)x);
DateFormat format = new SimpleDateFormat("HHmmss");
Date date = format.parse(s);
I don’t know how your double value is representing a date, but the code can solve your example question.
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solved How to convert a java double value to Date/Time format (HH:mm:ss) in Java?