Mark the date_time function from OffsetDateTime as public

[samrg472]

Hello,

At present, the best way to convert an OffsetDateTime to PrimitiveDateTime is to use the new function and construct it from parts. If OffsetDateTime::date_time was public, the conversion below wouldn't be necessary.

I use PrimitiveDateTime for database storage where columns are typically stored without a timezone for UTC.

let now = OffsetDateTime::now_utc();
let primitive = PrimitiveDateTime::new(now.date(), now.time());

[jhpratt]

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