# XO0420: constant std/time layout is invalid

A layout passed to `time.format`, `time.parse`, or `time.check_layout` is written with named, lowercase fields: `{year}`, `{month}` (also `{month:1}`, `{month:short}`, `{month:name}`), `{day}` (`{day:1}`), `{weekday:short}`, `{weekday:name}`, `{hour}`, `{hour12}` with `{ampm}`, `{minute}`, `{second}`, `{frac}` (`{frac:1}` to `{frac:9}`), `{offset}` (`{offset:compact}`), `{zone}`, and `{lbrace}`/`{rbrace}` for literal braces (std.md 13.3). When the layout is a constant (a string literal, a `const`, or a `+` of them) the checker parses it: an unknown field, a field form that does not exist, a stray brace, or `{hour12}` without `{ampm}` is this error, with a did you mean fix for a misspelled field. Date patterns of other languages (`{yyyy}`, `yyyy-MM-dd`, `%Y-%m-%d`, Go's `2006-01-02`) are the habit XO0177 instead, with a fix to the named fields. A layout built at run time is not checked here: `format` and `parse` fault on a bad one, and `time.check_layout` returns a `Result` for layouts that come from input. See decision 0094.

## Example

```xo
use std/time

fn stamp(t: Time) -> Str {
  time.format(t, time.utc(), "{year}-{month}-{dya}")
}
```

## Fix

```xo
use std/time

fn stamp(t: Time) -> Str {
  time.format(t, time.utc(), "{year}-{month}-{day}")
}
```

Run `xo explain XO0420` for this text in a terminal, or see
[section 10.6 of the specification](../../spec/core/#106-diagnostic-codes).