如何实现按年月计算日期及逆向推导，1月场景下的特殊日期范围需求

Alright, let's break down how to implement this date range logic clearly and concisely:

Core Requirements #

• Build two core functionalities:
  1. Compute the full start-to-end date range when given a specific year and month.
  2. Reverse-engineer the corresponding year and month from a provided single date.

Special Scenario Handling #

The behavior adjusts based on the current calendar month and the user's selected month:

• When current month is January
  If the user picks November or December, return the full date range of that month from the previous year.
• When current month is not January
  If the user selects the current month, return the full date range of that month in the current year.

Practical Examples #

• Example 1: Current date is 2018-01-01
  • User selects December → returns range: 2017-12-01 to 2017-12-31
  • User selects November → returns range: 2017-11-01 to 2017-11-30
• Example 2: Current date is 2018-12-15
  • User selects December → returns range: 2018-12-01 to 2018-12-31

内容的提问来源于stack exchange，提问作者JayV