This is an umbrella ticket for building Spark's own Datetime patterns and related works.
In Spark version 2.4 and earlier, datetime parsing and formatting are performed by the old Java 7 `SimpleDateFormat` API. Since Spark 3.0, we switch to the new Java 8 `DateTimeFormatter` to use the Proleptic Gregorian calendar, which is required by the ISO and SQL standards.
However, there are some datetime patterns not compatible between Java 8 and Java 7 APIs, and it's fragile to rely on the JDK API to define Spark's behavior. We should build our own Datetime patterns, which is compatible with Spark 2.4 (the old Java 7 `SimpleDateFormat` API).
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