Small simplifications

[pitdicker]

Some small simplifications to DateTime::date_naive and NaiveDate::diff_months, a commit from #1523 to replace .and_local_timezone(Utc).unwrap() with and_utc(), and a commit to remove some strange documentation from NaiveDateTime::and_local_timezone.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.81%. Comparing base (236b7ad) to head (4ca2940).
Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1530   +/-   ##
=======================================
  Coverage   91.81%   91.81%           
=======================================
  Files          40       40           
  Lines       18351    18319   -32     
=======================================
- Hits        16849    16820   -29     
+ Misses       1502     1499    -3     

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