{{ $content->updated_at->tz('UTC')->toAtomString() }}

{{ CarbonCarbon::parse($content->updated_at)->tz('UTC')->toAtomString() }}

@if ($content->updated_at)
  <lastmod>{{ ($content->updated_at)->tz('UTC')->toAtomString() }}</lastmod>
@endif

class Article extends Model
{
  protected $casts = [
    // ... другие поля.
    'created_at' => 'datetime',
    'updated_at' => 'datetime',
  ];
}