Human readable and parsable date format in Java

Question

I want to save a Date object to a readable string (for example 22/10/2009 21:13:14) that is also parsable back to a Date object.

I have tried many things and the best I could find was to use DateFormater for parsing and formating but it has a setback. When you format a date you lose seconds information. I tried to find if there is an option to format it and display the seconds (even better would be to the millisecond level since that's the resolution the Date object allows you to have) but I came up short.

Any ideas?

Harmonie

Answer 1

Take a look at java.text.SimpleDateFormat

SimpleDateFormat sdf = new SimpleDateFormat("MM/dd/yyyy HH:mm:ss.SSS");
Date dt = new Date();
String S = sdf.format(dt); // formats to 09/23/2009 13:53:28.238
Date dt2 = sdf.parse(S); // parses back

Halvorsen

When this answer was written in 2009, SimpleDateFormat was what we had for this purpose. It has proven troublesome and has fortunately been replaced by java.time, the modern Java date and time API, and its DateTimeFormatter class in 2014. I recommend you never use SimpleDateFormat again. – Rutilant

Why we must set the format of the provided string to SimpleDateFormat? Why SimpleDateFormat cannot determine the format itself? Because of the developers of SimpleDateFormat class was so lazy – Pearlene

Answer 2

SimpleDateFormat can format and parse a date based on a very simple pattern system that include second and even milliseconds.

Rohn

@Unihedron Updated the link. Reviewers, please note that this is NOT a link-only answer - the answer still has meaning without the link, and the link itself is to the official documentation. – Erythropoiesis

Answer 3

Other answers are all good.

But when doing this kind of thing please pick a format that sorts properly when coded as a string.... "yyyy/MM/dd HH:mm:ss" is fine. It always astounds me when software engineers pick a date format which doesn't sort in the obvious, convenient way.

You'll save your fellow developers a lot of pain at some distant point in the future - think of it as good karma :-)

Slake

"yyyy/dd/MM" doesn't sort right. Consider that 2015/21/10, 2015/20/11 and 2015/22/11 get sorted into 2015/20/11, 2015/21/10, 2015/22/11 as strings. – Apthorp

Sorry I meant yyyy/MM/dd. Fixed. – Slake

Answer 4

ISO 8601

Use ISO 8601 format.

• It’s flexible, it includes seconds and fraction of second if there are any, but you may also leave them out if they are 0.
• It’s standard, so more and more tools format and parse it. Great for serialization for storage or data interchange.
• It goes like 2009-10-22T21:13:14, I should say it’s pretty human-readable (though the T in the middle that denotes the start of the time part may feel unusual at first).
• The strings sort properly, as mikera requested in another answer, as long as the years are in the four-digit range from 1000 through 9999.
• The classes of java.time, the modern Java date and time API, as well as those of Joda Time parse ISO 8601 as their default, that is, without any explicit formatter, and produce the same format from their toString methods.

A modest demonstration of using java.time:

LocalDateTime dateTime = LocalDateTime.of(2009, 10, 22, 21, 13, 14);
String readableString = dateTime.toString();
System.out.println(readableString);
LocalDateTime parsedBack = LocalDateTime.parse(readableString);
System.out.println(parsedBack);

This prints two identical lines:

2009-10-22T21:13:14
2009-10-22T21:13:14

The latter System.out.println() call implicitly calls toString() once more, so this shouldn’t surprise.

Rutilant

The question is to convert Date object, if I want to use your code then I need to convert Date to LocalDateTime, how can I do it? – Glen

Thank you, @RoyHabeahan, for your interest. Given an old-fashioned Date do String readableString = yourDate.toInstant().toString(); and for the opposite way Date parsedBack = Date.from(Instant.parse(readableString));. The string will have Z in the end, it denotes UTC. Don't use LocalDateTime in this case. – Rutilant

Answer 5

A little off-topic, but I always feel the need to remind people that DateFormat and SimpleDateFormat are not thread safe! The Sun documentation clearly states this, but I keep finding code out in the wild where people stick a SimpleDateFormat in a static ...

Smashed

That's why everybody should use Joda Time instead, and save themselves headaches :) – Understate

I totally agree. On the other hand I want Sun to fix these classes, so if nothing else, I don't have to sit through yet another "concurrency lessons learned"-session with people warning me about them. (Oh the irony ... I just did exactly the same thing here on StackOverflow :-)) – Smashed

Oracle (the company that acquired Sun) has fixed it. The classes of java.time, the modern Java date and time API, are thread-safe. Just forget everything about the old SimpleDateFormat. – Rutilant

Answer 6

If you want to do it a little simpler, and be spared from making your own DateFormat that most other Answers involve, you can leverage the default format in java.time.Instant:

(new Date()).toInstant.toString();

Hayden