google/gson · error · IllegalArgumentException

Unknown DateFormat style: {dateStyle}

Error message

Unknown DateFormat style: {dateStyle}

What it means

Thrown by PreJava9DateFormatProvider.getDatePartOfDateTimePattern as IllegalArgumentException when the dateStyle argument is not one of DateFormat.SHORT, MEDIUM, LONG, or FULL. The provider only knows those four standard styles (the ones Java 8 used for US-locale date formatting); any other int (including DateFormat.DEFAULT or custom values) falls through to the default branch.

Source

Thrown at gson/src/main/java/com/google/gson/internal/PreJava9DateFormatProvider.java:47

   */
  public static DateFormat getUsDateTimeFormat(int dateStyle, int timeStyle) {
    String pattern =
        getDatePartOfDateTimePattern(dateStyle) + " " + getTimePartOfDateTimePattern(timeStyle);
    return new SimpleDateFormat(pattern, Locale.US);
  }

  private static String getDatePartOfDateTimePattern(int dateStyle) {
    switch (dateStyle) {
      case DateFormat.SHORT:
        return "M/d/yy";
      case DateFormat.MEDIUM:
        return "MMM d, yyyy";
      case DateFormat.LONG:
        return "MMMM d, yyyy";
      case DateFormat.FULL:
        return "EEEE, MMMM d, yyyy";
      default:
        throw new IllegalArgumentException("Unknown DateFormat style: " + dateStyle);
    }
  }

  private static String getTimePartOfDateTimePattern(int timeStyle) {
    switch (timeStyle) {
      case DateFormat.SHORT:
        return "h:mm a";
      case DateFormat.MEDIUM:
        return "h:mm:ss a";
      case DateFormat.FULL:
      case DateFormat.LONG:
        return "h:mm:ss a z";
      default:
        throw new IllegalArgumentException("Unknown DateFormat style: " + timeStyle);
    }
  }
}

View on GitHub (pinned to 8b8628c656)

Solutions

1. Restrict dateStyle to DateFormat.SHORT, MEDIUM, LONG, or FULL; validate before calling.
2. If you need a non-standard format, build a SimpleDateFormat with an explicit pattern instead of relying on the style-based factory.
3. Map any DEFAULT constant to MEDIUM before forwarding to the provider.

Example fix

// before
DateFormat f = PreJava9DateFormatProvider.getUsDateTimeFormat(99, DateFormat.SHORT); // throws

// after: use a valid constant or explicit pattern
DateFormat f = PreJava9DateFormatProvider.getUsDateTimeFormat(DateFormat.MEDIUM, DateFormat.SHORT);
// or
DateFormat f = new SimpleDateFormat("MMM d, yyyy h:mm a", Locale.US);

static void checkDateStyle(int style) {
  if (style != DateFormat.SHORT && style != DateFormat.MEDIUM
      && style != DateFormat.LONG && style != DateFormat.FULL) {
    throw new IllegalArgumentException("Unsupported date style: " + style);
  }
}

static boolean isValidDateStyle(int s) {
  return s == DateFormat.SHORT || s == DateFormat.MEDIUM
      || s == DateFormat.LONG || s == DateFormat.FULL;
}

try {
  PreJava9DateFormatProvider.getUsDateTimeFormat(dateStyle, timeStyle);
} catch (IllegalArgumentException e) {
  if (e.getMessage().contains("dateStyle")) {
    dateStyle = DateFormat.DEFAULT == dateStyle ? DateFormat.MEDIUM : DateFormat.MEDIUM;
  } else throw e;
}

When it happens

Trigger: Calling getUsDateTimeFormat(dateStyle, timeStyle) with an invalid dateStyle; reached internally by Gson when it builds a default date format for a style constant outside the supported set. Also fires if user code passes DateFormat.DEFAULT (value 2, which collides with MEDIUM only by coincidence) or a negative/arbitrary int.

Common situations: Passing a raw int for date style that is not a recognized DateFormat constant; using a custom style index; version/interop code that forwards an unchecked style value.

Related errors

• Unknown DateFormat style: {timeStyle}
• No time zone indicator
• Invalid time zone indicator {timezoneIndicator}
• Failed to parse date [{input}]: {fail.getMessage()}
• {value}

AI-assisted analysis of google/gson@8b8628c656 (2026-08-04). Data as JSON: /data/errors/406135e10f20c811.json. Report an issue: GitHub.