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

use std/time

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

Fix

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.