test(format/date-time): add high-precision second fraction as valid

[vtushar06]

Following the methodology I used for ipv4 and uuid, I read RFC 3339 section 5.6 and found the suite has no date-time test for a second fraction long enough to expose the float-coercion bound in a real validator.

RFC 3339 section 5.6 defines time-secfrac = "." 1*DIGIT - the fraction is unbounded, any number of digits is valid. 1985-04-12T00:59:59.999999999999999Z (second 59, fifteen 9s) is therefore valid. This exercises the boundary a validator hits if it coerces the seconds field to a float: fifteen 9s is the smallest count that rounds 59.999... up to exactly 60.0.

Changes

• Added 1 test case across draft3, draft4, draft6, draft7, draft2019-09, draft2020-12, and v1.
• 1985-04-12T00:59:59.999999999999999Z - a valid second fraction of fifteen 9s - valid.

Ecosystem Impact

1. ajv-formats 3.0.1 full: FAILS (rejects it). ajv computes sec = Number("59." + fifteen 9s), which is 60.0 exactly (IEEE-754; the gap between representable doubles at 60 is 2^-47), so sec < 60 is false and it falls into the leap-second branch, which needs UTC hour 23 - at hour 00 it rejects a valid input. This is the default addFormats(ajv) mode; ajv-fast (pure regex) accepts it. Fourteen 9s (the control) stays below 60.0 and is accepted.
2. python-jsonschema 4.25.1: PASSES (accepts it).
3. sourcemeta/core is_rfc3339_datetime: PASSES (accepts it).
4. Corroboration: jsonschema-rs 0.45.0, boon 0.6.1, santhosh-tekuri/jsonschema v6.0.2, and @hyperjump/json-schema 1.17.6 all accept it - ajv-formats full is the lone validator wrongly rejecting a valid input.

RFC References

• RFC 3339 section 5.6 (Internet Date/Time Format, time-secfrac = "." 1*DIGIT): https://www.rfc-editor.org/rfc/rfc3339#section-5.6

Reproduction and the date-time cross-implementation matrix are in my evidence repo: https://github.com/vtushar06/JSON-Schema-format-test-Evidence/blob/main/date-time.md

Related: #965

[Copilot]

Pull request overview

Adds a new format: "date-time" conformance test to ensure validators accept RFC 3339 second fractions with sufficiently high precision (15 fractional digits), helping catch implementations that incorrectly coerce seconds into IEEE-754 floats and mis-handle boundary rounding.

Changes:

• Added a valid date-time instance with a 15-digit fractional second (1985-04-12T00:59:59.999999999999999Z) to each supported test suite variant (draft3/4/6/7 optional, draft2019-09 optional, draft2020-12 optional, and v1).
• Ensures consistent coverage across all existing tests/**/format/date-time.json files in the repository.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
tests/v1/format/date-time.json	Adds a new valid high-precision fractional-second date-time test case.
tests/draft7/optional/format/date-time.json	Adds the same valid high-precision fractional-second date-time test case.
tests/draft6/optional/format/date-time.json	Adds the same valid high-precision fractional-second date-time test case.
tests/draft4/optional/format/date-time.json	Adds the same valid high-precision fractional-second date-time test case.
tests/draft3/optional/format/date-time.json	Adds the same valid high-precision fractional-second date-time test case.
tests/draft2020-12/optional/format/date-time.json	Adds the same valid high-precision fractional-second date-time test case.
tests/draft2019-09/optional/format/date-time.json	Adds the same valid high-precision fractional-second date-time test case.

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[jviotti]

Ha. Funny one!