Some timestamps are not parsed as such

[Giszmo]

I store timestamps as just dates as such:

const schema = yaml.DEFAULT_SCHEMA
schema.compiledTypeMap.scalar['tag:yaml.org,2002:timestamp'].represent = function (it) {
  return dateFormat(it, 'yyyy-mm-dd')
}
...
yaml.dump(header, {
  schema: schema
})

but somehow some values get handled right and others not.

The json contains:

released: '2021-08-26T07:00:00Z',
updated: '2023-03-25T12:43:55Z',

and my drop-the-hour logic handles the second value just fine but the first value not:

released: '2021-08-26T07:00:00Z'
updated: 2023-03-25

What might be wrong here?