Difference between hh:mm a and HH:mm a

Question

Here is my original code-

String dateString = "23 Dec 2015 1:4 PM";
Locale locale = new Locale("en_US");
SimpleDateFormat formatter = new SimpleDateFormat("dd MMM yyyy HH:mm a");
DateFormat df = new SimpleDateFormat("dd MMM yyyy HH:mm a", locale);
Date date = null;

try {
    date = formatter.parse(dateString);
} catch (ParseException e) {
    LOGGER.error(e);
}

String newDate = df.format(date);
System.out.println("oldDate = " + dateString);
System.out.println("newDate = " + newDate);

and here is my output-

oldDate = 23 Dec 2015 1:4 PM
newDate = 23 Dec 2015 01:04 AM

There is AM-PM difference between the oldDate and newDate. Now I changed the DateFormat code to-

SimpleDateFormat formatter = new SimpleDateFormat("dd MMM yyyy hh:mm a");
DateFormat df = new SimpleDateFormat("dd MMM yyyy hh:mm a", locale);

and I get the expected output, which is-

oldDate = 23 Dec 2015 1:4 PM
newDate = 23 Dec 2015 01:04 PM

I'm aware that HH signifies the 24 hour format and hh signifies the 12-hour format.

My question is

If I use HH:mm a instead of hh:mm a, shouldn't this be returning the time in 24-hour format?

(or)

If it defaults to 12-hour format, shouldn't it return respective AM/PM marker depending on the date input provided?

This is just for my understanding.

Answer 1

Updated Answer

Here the problem is with Program flow

SimpleDateFormat formatter = new SimpleDateFormat("dd MMM yyyy HH:mm a");

this simple date formatter is used to format the date string the Date object is created, here is the important part of answer

As you are using HH instead of hh, the SimpleDateFormater considers that provided date string is in 24-Hour Format and simply ignores the AM/PM marker here.

The Date Object from constructed from this SimpleDateFormatter is passed to the

DateFormat df = new SimpleDateFormat("dd MMM yyyy HH:mm a", locale);

That's why it's printing

newDate = 23 Dec 2015 01:04 AM

if you change the line

SimpleDateFormat formatter = new SimpleDateFormat("dd MMM yyyy HH:mm a");

with

SimpleDateFormat formatter = new SimpleDateFormat("dd MMM yyyy hh:mm a");

Everything will go smooth as it should!

Note : when creating a locale you should pass the language code to the Locale() constructor. en-us not en_us.

IMP : Code is tested on Java 8 also. Java(TM) SE Runtime Environment (build 1.8.0_121-b13)

Answer 2

Difference between hh:mm and HH:mm:

  HH:mm => will look like 00:12, 23:00... this has 24 hour format.

  hh:mm => will look like 01:00Am, 02:00Am,...01:00pm this has 12 hour format.

And the a in hh:mm a or in HH:mm a is Am/pm marker for more info go to link

Answer 3

A simple test is the best answer to how the pattern "HH:mm a" is interpreted. First let us investigate the printing mode:

The old format engine SimpleDateFormat:

Calendar cal = new GregorianCalendar(2015, 3, 1, 17, 45);
SimpleDateFormat sf = new SimpleDateFormat("HH:mm a", Locale.US);
System.out.println(sf.format(cal.getTime())); // 17:45 PM

Java-8 (with the new package java.time.format):

LocalTime time = LocalTime.of(17, 59);
DateTimeFormatter tf = DateTimeFormatter.ofPattern("HH:mm a", Locale.US);
System.out.println(tf.format(time)); // 17:59 PM

Here we don't observe any override in both cases. I prefer that approach because using the combination of "HH" and "a" is in my opinion rather an indication for a programming error (the user has obviously not thought enough about the meaning and official description of those pattern symbols).

Now let us investigate the parsing mode (and we will use strict mode to observe what is really going behind the scene):

String dateString = "23 Dec 2015 1:4 PM";
SimpleDateFormat df = new SimpleDateFormat("dd MMM yyyy H:m a", Locale.US);
df.setLenient(false);
Date date = df.parse(dateString);
// java.text.ParseException: Unparseable date: "23 Dec 2015 1:4 PM"

How is the behaviour in Java-8? It is the same but with a clearer error message:

DateTimeFormatter tf = DateTimeFormatter.ofPattern("H:m a", Locale.US);
tf = tf.withResolverStyle(ResolverStyle.STRICT);
LocalTime.parse("1:4 PM", tf);
// DateTimeException: Cross check failed: AmPmOfDay 0 vs AmPmOfDay 1

This message tells us that the parsed hour (in 24-hour format!) with value "1" indicates AM while the text input contains the other parsed AM/PM-value "PM". This ambivalence cannot be resolved.

Lesson: We have to be careful with lenient parsing where contradictious information can be ignored and lead to false assumptions. The accepted answer of @Arjun is completely wrong.

By the way: Please use Locale.US or new Locale("en", "US") instead of new Locale("en-US") because the language "en-US" does not exist.