ユリウス通日とDate型の相互変換

// julianday to Date
Date date = new Date((long)((julianday - 2440587.5) * 86400000));

// Date to julianday
float julianday = date.getTime() / 86400000 + 2440587.5;

※juliandayがfloat型の想定

UNIX時間(1970-01-01 00:00:00)をユリウス日で表した値=2440587.5