GROQ dateTime() function with Date type returning null

The dateTime() function in GROQ expects a datetime string in RFC3339 format (like 1985-04-12T23:20:50.52Z), which is why you're getting null when using it with a Date type field.

The issue is that Sanity's Date type stores only the date portion (like 2024-01-15) without the time component, so dateTime() can't parse it directly.

The fix: Concatenate a time string to your date field to make it RFC3339 compatible:

*[_type == "exhibition"]{
  "dateTime": dateTime(startDate + 'T00:00:00Z')
}

This appends midnight UTC to your date, converting it into a format that dateTime() can parse.

For comparing dates in a range:

You can use this approach to filter exhibitions within a date range:

*[_type == "exhibition" 
  && dateTime(startDate + 'T00:00:00Z') >= dateTime("2024-01-01T00:00:00Z")
  && dateTime(startDate + 'T00:00:00Z') <= dateTime("2024-12-31T23:59:59Z")
]

Alternative approach: If you're just comparing dates (not times), you might not even need dateTime() at all. Simple string comparison works for date fields since they're stored in ISO format:

*[_type == "exhibition" 
  && startDate >= "2024-01-01"
  && startDate <= "2024-12-31"
]

This is cleaner and works perfectly fine for date-only comparisons. Save dateTime() for when you actually need datetime precision with time components.

The GROQ functions documentation has more details on how dateTime() works with different input formats.