Date Patterns Cheatsheet: Moment.js and Java’s SimpleDateFormatter

More and more projects seem to abandon Java as a front-end programming language in favor of JavaScript. Usually, they keep with Java as the backend programming language. Most of the time, both ecosystems exist peacefully. But there are sources of confusion. One of them being dates and times.

Does this sound familiar? Yes, you’re right. Dates and times are a pain point of Java since day one. JavaScript just adds another layer of complexity to this.

Moment.js is a very popular library to deal with dates and times in JavaScript. So I’ve assembled an interactive cheatsheet helping you to convert the formats into each other.

Similarities and differences

At first glance, the date formats of Java and moment.js are pretty much exchangeable. The pattern to format the date in German locale is “DD.MM.YY HH:mm:ss” in moment.js and “dd.MM.yy HH:mm:ss” in Java. That’s pretty much the same. Except for the capitalization.

And that’s precisely what make the conversion between moment.js and Java such an unpleasant experience. The concepts are the same, many formatting tokens are the same – except for subtle differences. Basically, you have to be careful with the capitalization and the number of characters. For instance, “D” prints the number of the day in the year in Java, but the number of the day in the month in moment.js. The moment.js counterpart of Java’s “D” is “DDD”. Adding insult to injury, “DDD” also denotes the number of the day of the year in Java – but it’s the three digit, zero-padded version. Which is “DDDD” in moment.js.

Formats that can’t be converted

There are also format strings that can’t be converted. For example, Java’s “G” denotes the era (i.e. A.D. or B.C.). There’s no counterpart in moment.js. Luckily, the common use cases are covered.

Sometimes, even the common use cases have surprises in store. The abbreviated versions of the names of the days and months are identical in both Java and moment.js. More precisely: they are identical in English. The Spanish, French and German translations add a dot in Java, but not in moment.js.

Predefined formats

Both moment.js and Java’s SimpleDateFormat have predefined formats for the most common use cases. Most of them don’t translate directly into each other. The cheatsheet covers the predefined formats, too. However, this translation table isn’t perfect, and it can’t be: Java defines differerent predefined formats for different locales.

Escaping characters

Both moment.js and SimpleDateFormat allow you to add non-format strings, such as spaces, commas, slashes and dashes. These strings are added directly to the formatted date. You can also escape characters. In moment.js characters are escaped by surrounding them with square brackets. SimpleDateFormat does the same by putting the string into single quotes:

// Java:
new SimpleDateFormat("EEE, d MMM yyyy 'at' hh:mmA")
    .format(new Date());
// JavaScript:
moment().format("ddd, D MMM YYYY [at] hh:mmA");

Both will output something like “Wed, 26 Mar 2017 at 11:17AM”.

Programmatic conversion

There’s a slightly buggy JavaScript project: We’ve used it a basis for the conversion needed by the BootsFaces project (and removed the bugs along the way). So you can find a Java version of the converter at the BootsFaces GitHub repository.

Date patterns cheatsheet

Follow this link to get to the interactive cheatsheet.

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