converting epoch to ZonedDateTime in Java

Question

How to convert epoch like 1413225446.92000 to ZonedDateTime in java?

The code given expects long value hence this will throw NumberFormatException for the value given above.

ZonedDateTime.ofInstant(Instant.ofEpochMilli(Long.parseLong(dateInMillis)), ZoneId.of(TIME_ZONE_PST));

Answer 1

Split the number into a pair of 64-bit long integers:

• Number of whole seconds since the epoch reference date of first moment of 1970 in UTC
• A number of nanoseconds for the fractional second

Pass those numbers to the factory method Instant.ofEpochSecond​(long epochSecond, long nanoAdjustment)

With an Instant in hand, proceed to assign a time zone to get a ZonedDateTime.

ZoneId z = ZoneId.of( "America/Los_Angeles" ) ;
ZonedDateTime zdt = instant.atZone( z ) ;

Answer 2

Basil Bourque’s answer is a good one. Taking out the nanoseconds from the fractional part into an integer for nanoseconds may entail a pitfall or two. I suggest:

    String dateInMillis = "1413225446.92000";
    String[] secondsAndFraction = dateInMillis.split("\\.");
    int nanos = 0;
    if (secondsAndFraction.length > 1) { // there’s a fractional part
        // extend fractional part to 9 digits to obtain nanoseconds
        String nanosecondsString
                = (secondsAndFraction[1] + "000000000").substring(0, 9);
        nanos = Integer.parseInt(nanosecondsString);
        // if the double number was negative, the nanos must be too
        if (dateInMillis.startsWith("-")) {
            nanos = -nanos;
        } 
    }
    ZonedDateTime zdt = Instant
            .ofEpochSecond(Long.parseLong(secondsAndFraction[0]), nanos)
            .atZone(ZoneId.of("Asia/Manila"));
    System.out.println(zdt);

This prints

2014-10-14T02:37:26.920+08:00[Asia/Manila]

We don’t need 64 bits for the nanoseconds, so I am just using an int.

If one day your epoch time comes in scientific notation (1.41322544692E9), the above will not work.

Please substitute your desired time zone in the region/city format if it didn’t happen to be Asia/Manila, for example America/Vancouver, America/Los_Angeles or Pacific/Pitcairn. Avoid three letter abbreviations like PST, they are ambiguous and often not true time zones.