JSON Schema validation for ISO 8601 date strings

Direct answer

Set "type": "string" together with "format": "date" for calendar dates (YYYY-MM-DD) or "format": "date-time" for timestamps. However, the format keyword is an annotation in most JSON Schema drafts unless the validator is explicitly configured to enforce it.

Schema configuration

{
  "type": "string",
  "format": "date"
}

For a full timestamp:

{
  "type": "string",
  "format": "date-time"
}

Constraints across schema versions

The format keyword has existed since Draft 4 and continues through Drafts 6, 7, 2019-09, and 2020-12. In all of these, format validation is optional by default; the specification does not require validators to reject strings that fail the format assertion.

Implementation-specific enforcement

• AJV (JavaScript): format is ignored unless you pass validateFormat: true (or enable the appropriate format vocabulary in newer drafts).
• Python jsonschema: you must attach a FormatChecker to the validator instance; otherwise format is treated as a no-op.
• Java validators (e.g., Everit, networknt): behavior varies; some enforce date and date-time out of the box, others require a configuration flag or library switch.

Fallback when format is not enforced

If you cannot rely on the validator's format support, add a pattern that matches the ISO 8601 shape:

{
  "type": "string",
  "pattern": "^\\d{4}-\\d{2}-\\d{2}$"
}

Be aware that a regex only checks structure, not calendar validity. A value such as 2026-02-30 may pass the pattern even though it is not a real date. Some validators perform range checks when format enforcement is active, but this is implementation-dependent.

Verification

Confirm that your chosen validator actually enforces the constraint. A minimal Python check looks like this:

from jsonschema import validate, FormatChecker

schema = {"type": "string", "format": "date"}
validate("2026-04-15", schema, format_checker=FormatChecker())  # passes
validate("2026-13-01", schema, format_checker=FormatChecker())  # raises ValidationError

Repeat with "format": "date-time" and a value like "2026-04-15T12:30:45Z" to confirm timezone and fractional-second handling.

If you need the exact configuration flag for a specific validator library, let me know which one you are using.