Detect if date contains at least a full month (1st to last day of month)

containsfullmonth.cs

using System;

public class Program
{
	public static void Main(string[] args)
	{
		DateTime start = new DateTime(2020, 12, 15);
		DateTime end = new DateTime(2021, 1, 20);
		System.Diagnostics.Debug.WriteLine(IsFullMonth(start, end));
		System.Diagnostics.Debug.Close();
	}

	private static bool IsFullMonth(DateTime start, DateTime end)
	{
		int start_firstday = 1;
		int end_lastday = DateTime.DaysInMonth(end.Year, end.Month);

		//2021 01 15 - 2021 03 15
		if (end.Month - start.Month > 1)
			return true;

		//2021 03 01 - 2021 03 31
		else if (end.Month - start.Month == 0 && end.Year == start.Year)
		{
			if (start.Day == start_firstday && end.Day == end_lastday)
				return true;
			return false;
		}

		//end.Month - start.Month >= 1   
		else
		{
			//2020 02 01 - 2021 03 31
			//2020 12 01 - 2021 01 31
			if (start.Day == start_firstday || end.Day == end_lastday)
				return true;

			//2020 07 15 - 2021 08 20
			else if (start.Year < end.Year && start.Month != 12)
				return true;

			//2020 12 15 - 2021 08 22
			//2020 12 02 - 2021 02 15
			else if (start.Year < end.Year && start.Month - end.Month < 11)
				return true;
			
			return false;
		}
	}
}

Some unions calculate their members eligibility this way