Date format converter

Why the same letter means different things

Format strings look interchangeable and are not. The clearest trap is the letter m: in PHP's date() it is the two-digit month, while in Java, C# and moment it is the minute — so copying Y-m-d into Java as y-m-d silently prints the minute where you wanted the month, and the bug only looks wrong for part of each hour. Java and moment disagree about case instead: the day of the month is dd in Java but DD in moment, where DD means day of year in Java. Translating through the meaning of each field, rather than letter by letter, is the only reliable way across.

Go's reference time

Go does not use letters at all. A layout is written as the specific reference instant Mon Jan 2 15:04:05 MST 2006, and each component you include is matched by its value: 2006 is the four-digit year, 01 the month, 02 the day, 15 the 24-hour hour, 04 minutes and 05 seconds. The numbers are mnemonic — 1, 2, 3, 4, 5, 6 in the order month, day, hour, minute, second, year — which is why 2006-01-02 15:04:05 is the Go spelling of yyyy-MM-dd HH:mm:ss.

Excel's ambiguous m

In Excel custom number formats, m means month or minute depending on where it sits: directly after an h, or immediately before an s, it is read as minutes, and anywhere else as the month. So hh:mm is hours and minutes while yyyy-mm is year and month, with the same two letters. This page applies the same rule when reading an Excel pattern, so the translation to other languages comes out right.

What runs where

The strftime column covers C, Python's strftime, Ruby and the Unix date command. Java's column matches both SimpleDateFormat and the newer DateTimeFormatter, which Kotlin also uses. The moment column applies to Day.js and most JavaScript formatting libraries that inherited moment's tokens. Everything is computed in your browser; nothing is sent anywhere.