# XO0916: a string time layout parsed on every call

Reported by `xo vet`. `time.format(t, z, "{year}-{month}")` and `time.parse` walk the layout text on every call: 5.4 and 4.7 of Go's wall, against 2.0 and 1.9 for the methods of a `const Layout` (decision 0332), which parses it once. Declare the layout as a constant and call its method. A layout built at run time is not reported, and a cold call can say why with `// xo:allow XO0916 REASON` on the line or the line above.

## Example

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

## Fix

```xo
const DAY: time.Layout = time.layout("{year}-{month}-{day}")

fn day(t: time.Time, z: time.Zone) -> Str {
  DAY.format(t, z)
}
```

Run `xo explain XO0916` for this text in a terminal, or see
[section 10.6 of the specification](../../spec/core/#106-diagnostic-codes).