Add YearMonth scalar

Author: ayushvora10

scalars/contributed/ayushvora10/year-month.md

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+<!-- cspell:ignore ayushvora10 -->
+
+# YearMonth — GraphQL Custom Scalar
+
+"Author - ayushvora10"
+
+"Date - 2024-07-27"
+
+**License and Copyright**
+
+Copyright © GraphQL contributors. This specification is licensed under
+[OWFa 1.0](https://www.openwebfoundation.org/the-agreements/the-owf-1-0-agreements-granted-claims/owfa-1-0).
+
+# Overview
+
+This string-based scalar represents a specific month of a specific year (not just any month) following ISO 8601.
+
+The pattern is “YYYY-MM” with “YYYY” representing the year and “MM” the month.
+
+This can be used for records that are monthly in nature. Eg. a monthly sales forecast.
+
+# Name
+
+`YearMonth` inspired by [Java](https://docs.oracle.com/javase/8/docs/api/java/time/YearMonth.html)
+
+Or `ISO8601YearMonth`
+Or just `Month`?
+`ISO8601Month`?
+
+# Result spec
+
+The result must be an ISO 8601 extended format string with just the year and month: “YYYY-MM”
+
+## Valid examples
+
+| String    | Explanation |
+| -------- | ------- |
+| `2024-07`  | Representing July 2024    |
+| `2024-12` | Representing December 2024     |
+| `0999-01`    | Representing January of the year 999    |
+
+## Invalid examples
+
+| String    | Explanation |
+| -------- | ------- |
+| `2024-7`  | Zero-padding missing on the month, should be 2024-07 |
+| `24-07` | Truncated representations on year (YY instead of YYYY) are not recommended by ISO and can be confusing |
+| `2024-07-01`    | Full date is not expected, precision must only be up to month |
+| `2024-07-01T00:00:00Z`    | Full date-time is not expected, precision must only be up to month |
+| `07-2024`    | Should be YYYY-MM, not the other way |
+| `2024-00`    | Month must be 1-indexed, not 0-indexed: Jan = 01, Dec = 12 |
+| `202407`    | Hyphen missing; ISO recommends the “basic format (one without hyphens) should be avoided in plain text” |
+| `2024/07`    | Slash instead of hyphen |
+| `2024`    | Incomplete - month missing |
+| `2024-Jul`    | No strings allowed |
+| `+2024-07`    | No prefix (or postfix) symbols or characters allowed |
+
+# Input spec
+
+For input, _any valid ISO 8601 string_ is allowed.
+
+## Valid examples
+
+| String    | Explanation |
+| -------- | ------- |
+| `2024-07`  | Representing July 2024    |
+| `2024-07-13` | Still representing July 2024 - the implementation can ignore the date as needed     |
+| `2024-07-13T16:23:58Z` | Still representing July 2024 - the implementation can ignore everything beyond month as needed     |
+| `2024`  | Just year is valid ISO and must be treated as January of that year |
+
+## Invalid examples
+
+| String    | Explanation |
+| -------- | ------- |
+| `2024-July`  | Invalid ISO |
+
+# References
+
+- [ISO 8601](https://en.wikipedia.org/wiki/ISO_8601)
+List external references, other background information etc.