var countdown = new TimerRemaining()
{
    buffer =
    {
        [0] = 0,
        [1] = 1,
    }
};

var countdown = new TimerRemaining()
{
    buffer =
    {
        [0..3]  = 0,
        [4..5]  = 1,
        [6..^1] = 2,
    }
};

https://learn.microsoft.com/en-us/dotnet/csharp/whats-new/csharp-13#implicit-index-access

The implicit "from the end" index operator, ^, is now allowed in an object initializer expression.
...
The preceding example creates an array that counts down from 9 to 0. In versions before C# 13, the ^ operator can't be used in an object initializer. You need to index the elements from the front.

IT>var countdown = new TimerRemaining()
IT>{
IT>    buffer =
IT>    {
IT>        [0..3]  = 0,
IT>        [4..5]  = 1,
IT>        [6..^1] = 2,
IT>    }
IT>};
IT>

        var ex = new Ex
        {
            Buffer =
            {
             [0..3]  = 0,
             [4..5]  = 1,
             [6..^1] = 2,            
            }
        };
        var c = ex.Buffer.Slice(0, ^2); // ex.Buffer[0..^2] now doesn't work