How do I make calculation on a custom Epoch time? Not the UNIX epoch

Question

I am receiving data from a server in seconds and I want to convert it to date.

But the seconds I am receiving is not since the UNIX epoch 01/01/1970 but is 01/01/2000.

Normally I'd use:

SimpleDateFormat formatter = new SimpleDateFormat("dd/MM/yyyy", Locale.US);
String dateString = formatter.format(new Date(millisToConvert));

Where millisConvert is converted-to-milliseconds seconds that I receive. But now, naturally, I get wrong dates since the origin is different. How do I change the epoch (the origin) ?

Answer 1

tl;dr

OffsetDateTime.of( 2000 , 1 , 1 , 0 , 0 , 0 , 0 , ZoneOffset.UTC )  // Define your epoch reference moment. 
.pluSeconds( yourSeconds )  // Add the number of seconds given to you. Returns another `OffsetDateTime` object rather than altering the original (immutable objects pattern).  
.toLocalDate()     // Extract a date-only value without time-of-day and without offset-from-UTC. 

java.time

Use the modern java.time classes that supplanted the terrible old legacy classes such as Date & Calendar.

Instant

An Instant represents a moment in UTC.

Define your particular epoch reference date-time.

Instant epoch = Instant.parse( "2000-01-01T00:00Z" ) ;

Add your number of seconds.

Instant later = epoch.plusSeconds( yourSecondsGoHere ) ; 

OffsetDateTime

If you want a date-only value without time-of-day and without time zone, use LocalDate class. First, convert from the basic type Instant to the more flexible OffsetDateTime, specifying the constant ZoneOffset.UTC as the offset.

OffsetDateTime odt = instant.atOffset( ZoneOffset.UTC ) ;

LocalDate

Extract the date-only value.

LocalDate ld = odt.toLocalDate() ;

---

About java.time

The java.time framework is built into Java 8 and later. These classes supplant the troublesome old legacy date-time classes such as java.util.Date, Calendar, & SimpleDateFormat.

The Joda-Time project, now in maintenance mode, advises migration to the java.time classes.

To learn more, see the Oracle Tutorial. And search Stack Overflow for many examples and explanations. Specification is JSR 310.

You may exchange java.time objects directly with your database. Use a JDBC driver compliant with JDBC 4.2 or later. No need for strings, no need for java.sql.* classes.

Where to obtain the java.time classes?

• Java SE 8, Java SE 9, Java SE 10, Java SE 11, and later
  • Built-in.
  • Part of the standard Java API with a bundled implementation.
  • Java 9 adds some minor features and fixes.
• Java SE 6 and Java SE 7
  • Much of the java.time functionality is back-ported to Java 6 & 7 in ThreeTen-Backport.
• Android
  • Later versions of Android bundle implementations of the java.time classes.
  • For earlier Android (<26), the ThreeTenABP project adapts ThreeTen-Backport (mentioned above). See How to use ThreeTenABP….

The ThreeTen-Extra project extends java.time with additional classes. This project is a proving ground for possible future additions to java.time. You may find some useful classes here such as Interval, YearWeek, YearQuarter, and more.

Answer 2

Add 946080000 seconds to account for the 30 year difference, epoch time should be an long value

Answer 3

Another way apart from what Basil suggested as I marked as an answer. What Basil suggests works only on API level 26 and higher meaning Android 8.0.0. Nowadays, post-mid 2018, Android 8.0.0 is pretty rare so many devices will not have this functionality.

So I wrote this algorithm:

private long getDateInMilliSeconds (long secondsSinceRippleEpoch){

        Locale.setDefault(Locale.US);

        if (Build.VERSION.SDK_INT >= 26) { //**WHAT BASIL SUGGESTED**
            LocalDateTime desiredDate = OffsetDateTime.of(2000, 1, 1, 0, 0, 0, 0, ZoneOffset.UTC)
                    .plusSeconds(secondsSinceRippleEpoch)
                    .toLocalDateTime();

            Date convertedDate = Date.from(desiredDate.atZone(ZoneId.systemDefault()).toInstant());

            return convertedDate.getTime();
        } else { //MY SOLUTION. Works for both but still I wanted to use the upper
//for higher APIs.

            SimpleDateFormat formatter = new SimpleDateFormat("dd/MM/yyyy HH:mm:ss", Locale.US);
            String dateString = formatter.format(new Date((secondsSinceRippleEpoch+946684800)*1000L )); //To not to cause overflow, use L at the end.

            try {
                Date wantedDate = formatter.parse(dateString);
                return wantedDate.getTime();
            } catch (ParseException e) {
                e.printStackTrace();
                return 200000;
            }

        }

Hope this helps the future visitors.