Converting unix timestamp to datetime in Java

Question

I have an Unix timestamp (with microsecond precision) value in 1.514408397505346E9. What is the correct way to convert it to readable date time format?

Is it correct to convert like below?

java.util.Date dateTime = new java.util.Date((long) Double.valueOf(unixTimestamp).longValue() * 1000);
DateFormat df = new SimpleDateFormat("dd/MM/yyyy HH:mm:ss");
final String reportDate = df.format(dateTime);

Answer 1

java.time

You are using troublesome old date-time classes that are now legacy, supplanted by the java.time classes of Java 8 and later. The Date class in incapable of representing the microseconds resolution of your input data.

I assume your number represents a number of whole seconds + fractional second since the epoch reference date of first moment of 1970 in UTC, 1970-01-01T00:00Z. We need to tear apart the count of whole seconds from the fractional second. We want the fractional second as a count of nanoseconds.

We use BigDecimal for accuracy, as double / Double uses floating-point technology to trade away accuracy for performance.

String input = "1.514408397505346E9";
BigDecimal bd = new BigDecimal( input );
BigDecimal microsDivisor = new BigDecimal( 1_000_000_000L );
long seconds = bd.longValue( );
BigDecimal bdMicros = bd.subtract( new BigDecimal( seconds ) ).multiply( microsDivisor );
long nanos = bdMicros.longValue( );

input: 1.514408397505346E9

bd: 1514408397.505346

seconds: 1514408397

nanos: 505346000

From that, instantiate an Instant . TheInstant class represents a moment on the timeline in UTC with a resolution of nanoseconds (up to nine (9) digits of a decimal fraction).

Instant instant = Instant.ofEpochSecond( seconds , nanos ) ;

instant.toString(): 2017-12-27T20:59:57.505346Z

See this code run live at IdeOne.com .

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ISO 8601

This use of 1.514408397505346E9 is a terrible way to represent date-time values. When serializing date-time values, use the standard ISO 8601 formats only. In this case: 2017-12-27T20:59:57.505346Z

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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 .

Where to obtain the java.time classes?

• Java SE 8 , Java SE 9 , 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
  • The ThreeTenABP project adapts ThreeTen-Backport (mentioned above) for Android specifically.
  • 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

java.util.Date

Date currentTime = new Date(11111111111L);
SimpleDateFormat formatter = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
String dateString = formatter.format(currentTime);

And be careful, SimpleDateFormat is not threadsafe,