Convert a date range to an interval description
This answer shows an implementation using a SQL Server (2005+) CLR function.

-- Enable CLR (if necessary)
EXECUTE sys.sp_configure 
    @configname = 'clr enabled',
    @configvalue = 1;

RECONFIGURE;


Assembly and function

CREATE ASSEMBLY DBA
AUTHORIZATION dbo
FROM 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
WITH PERMISSION_SET = SAFE;
GO
CREATE FUNCTION dbo.IntervalDescription
(
    @From date, 
    @To date
)
RETURNS nvarchar(100)
AS EXTERNAL NAME 
    DBA.UserDefinedFunctions.IntervalDescription;


Usage

SELECT 
    TD.FromDate,
    TD.ToDate,
    TD.ExpectedResult, 
    IntervalDescription = dbo.IntervalDescription(TD.FromDate, TD.ToDate) 
FROM dbo.TestData AS TD;


Result





Source

I am not a C# programmer!

using Microsoft.SqlServer.Server;
using System;
using System.Text;

public partial class UserDefinedFunctions
{
    [SqlFunction
        (
        DataAccess = DataAccessKind.None,
        SystemDataAccess = SystemDataAccessKind.None,
        IsDeterministic = true,
        IsPrecise = true,
        Name = "IntervalDescription"
        )
    ]
    [return: SqlFacet(IsFixedLength = false, IsNullable = false, MaxSize = 100)]
    public static string IntervalDescription(DateTime From, DateTime To)
    {
        var workDate = From;
        int years = To.Year - From.Year;
        int months = 0;
        int days = 0;

        if (years != 0)
        {
            if (From.Month &gt; To.Month || (From.Month == To.Month &amp;&amp; From.Day &gt; To.Day))
            {
                years--;
            }
            workDate = workDate.AddYears(years);
        }

        while (workDate &lt; To &amp;&amp; (workDate.Year != DateTime.MaxValue.Year || workDate.Month != DateTime.MaxValue.Month))
        {
            if (workDate.AddMonths(1) &lt;= To)
            {
                months++;
                workDate = workDate.AddMonths(1);
            }
            else
            {
                break;
            }
        }

        while (workDate &lt; To)
        {
            days++;
            workDate = workDate.AddDays(1);
        }

        StringBuilder sb = new StringBuilder(100);

        if (years &gt; 0)
        {
            sb.Append(years);
            sb.Append(years == 1 ? " year" : " years");
            sb.Append((months &gt; 0 || days &gt; 0) ? ", " : string.Empty);
        }

        if (months &gt; 0)
        {
            sb.Append(months);
            sb.Append(months == 1 ? " month" : " months");
            sb.Append(days &gt; 0 ? ", " : string.Empty);
        }

        if (days &gt; 0 || (years == 0 &amp;&amp; months == 0))
        {
            sb.Append(days);
            sb.Append(days == 1 ? " day" : " days");
        }

        return
            sb.ToString();

    }
}