Sanity date field dateFormat option not being respected

The issue you're experiencing isn't due to a change in how the dateFormat option works - there's actually a mismatch between what you're expecting and what's happening.

According to the official Sanity documentation for the date type, the date type should store values as date-only strings in "YYYY-MM-DD" format, not as full ISO datetime strings with time components.

The fact that you're seeing "2021-04-01T07:00:00.000Z" (with time) instead of "2021-04" or "2021-04-01" suggests you might actually be using the datetime type instead of the date type, or there's a type mismatch somewhere in your schema.

Here's what's likely happening:

The dateFormat option only controls how the date is displayed in the Studio UI - it doesn't change what gets stored in your dataset. The stored value for a date type should always be "YYYY-MM-DD" format.

Your older documents with "2021-04" are storing incomplete date strings, which isn't standard for Sanity's date type. This might have been from a custom input or a different field type.

To fix this:

1. Double-check your schema - Make sure you're using type: 'date' and not type: 'datetime':

   {
     type: 'date',  // NOT 'datetime'
     name: 'date',
     options: {
       dateFormat: 'YYYY-MM'  // This only affects display, not storage
     }
   }

2. If you truly need year-month only storage, you'll need to use a custom input component or store it as a string type, since the native date type will always store the full "YYYY-MM-DD" format.

3. Check your queries/frontend - The datetime values you're seeing might be coming from a different field or being transformed somewhere in your data pipeline.

The dateFormat: 'YYYY-MM' option will make the Studio display the date as "2021-04", but the stored value will still be "2021-04-01" (with a day component defaulting to 01).