End date automatically defaults to first day of month instead of last day

Problem/Motivation
Currently, when an end date exists in the system, it automatically defaults to the first day of the month. This behavior is causing filtering issues in other parts of the application. We need to modify this functionality to default to the last day of the month instead.
Proposed resolution
Change the default end date behavior to set the date to the last day of the month rather than the first day.
Steps to reproduce

Navigate to content that contains an end date field
Observe that when an end date exists, it is automatically set to the first day of the month

Current behavior
The system sets the end date to the first day of the month.
Expected behavior
The system should set the end date to the last day of the month.