How to format a date in Java with SimpleDateFormat?

To create and use SimpleDateFormat, you need to pass in a format string, such as new SimpleDateFormat("yyyy-MM-dd HH:mm:ss"); 2. Pay attention to case sensitivity and avoid misuse of mixed single-letter formats and YYYY and DD; 3. SimpleDateFormat is not thread-safe. In a multi-thread environment, you should create a new instance or use ThreadLocal every time; 4. When parsing a string using the parse method, you need to catch ParseException, and note that the result does not contain time zone information; 5. It is recommended to use DateTimeFormatter and LocalDateTime instead for Java 8 and above.

Using SimpleDateFormat to format dates in Java is a very common operation, but it needs to be aware of its usage and some potential pitfalls. Here are some questions and practical suggestions that you may be concerned about.

How to create and use SimpleDateFormat?

To format a date, you first need to create a SimpleDateFormat instance and pass in the date format you want. For example:

 SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
String formattedDate = sdf.format(new Date());

The above code will output a string similar to 2025-04-05 14:30:00 .
The format string "yyyy-MM-dd HH:mm:ss" here is the key, where:

• yyyy indicates year
• MM indicates month (note that it is capitalized)
• dd represents date
• HH means 24-hour hours
• mm means minutes
• ss means seconds

You can freely combine these parts according to your needs, such as "yyyy/MM/dd" or "HH:mm" .

Common format errors and precautions

There are several error-prone places when using it:

• Case sensitivity : hh is 12-hour system, while HH is 24-hour system; mm is minutes, and MM is month.
• Do not mix single letter formats : for example, y represents the year, but in reality, it is recommended to use yyyy to avoid ambiguity.
• Thread Safety Issues : SimpleDateFormat is not thread-safe. If you use it in a multi-threaded environment, it is best to create a new instance every time or use ThreadLocal wrapper.

For example, the following format is prone to errors:

 new SimpleDateFormat("YYYY-mm-DD")

YYYY and DD are used here. Although they look like "year, month and day", in fact, YYYY is the "year of the anniversary" (unlike yyyy ), DD is the first day of the year (not the day of the month). This error will cause the results to not meet expectations.

How to parse a string as a date?

In addition to formatting, SimpleDateFormat can also parse strings into Date objects:

 String input = "2025-04-05 14:30:00";
SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
Date date = sdf.parse(input);

But be aware:

• If the input format does not match, ParseException will be thrown
• The parsed Date object is "original time point" without time zone information
• If the input contains information such as AM/PM, add a to the format to ensure the locale is consistent

Alternative: Java 8 and above are recommended to use java.time

If you are using Java 8 or later, it is recommended to use classes from the java.time package, such as DateTimeFormatter and LocalDateTime , which are more modern, thread-safe and have a clearer API:

 DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss");
String formatted = LocalDateTime.now().format(formatter);

Although this is beyond SimpleDateFormat , it is more worth recommending in new projects.

Basically that's it. As long as you remember the rules of format strings and thread safety issues, it will not be difficult to use.

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