balance.cs

public class Balance
{
    public Balance(decimal amount, DateTime asOf)
    {
        Amount = amount;
        AsOf = asOf;
    }

    public decimal Amount { get; private set; }
    public DateTime AsOf { get;private set; }

    public void Add(decimal value)
    {
        AsOf = DateTime.UtcNow;
        Amount += value;
    }

    public void Subtract(decimal value)
    {
        AsOf = DateTime.UtcNow;
        Amount -= value;

    }
}