Question

I’m working with the Excel format where

0.6549

Means that we are 65.49% through the day. How would I convert this to a nice looking time such as

15:30:42 PM

in Java?

Answer 1

Try this:

public static void main(String[] args) throws Exception {
    DateFormat format = new SimpleDateFormat("HH:mm:ss a");
    format.setTimeZone(TimeZone.getTimeZone("UTC"));
    for(String fracStr : args) {
        double frac = Double.parseDouble(fracStr);
        long day = (long) Math.floor(frac);
        frac = frac - day;
        Date time = new Date((long) ( 86400000l * frac));
        System.out.println(fracStr+" -> "+format.format(time));
    }
}

A little note here – the GMT time zone may not get you the results you expect, but UTC will. This is because Java considers GMT as a valid abbreviation for Europe/London and between 1968 and 1972 London was on permanent daylight saving. So midnight 1st January 1970 GMT (the Java epoch) is also 1am 1st January 1970 (GMT).