.NET approximate equality of two dates by determining if they are within one second of each other.

DateTimeExtensions.cs

using System;

namespace Extensions
{
    public static class DateTimeExtensions
    {
        public static bool ApproximatelyEqual(this DateTime t, DateTime obj)
        {
            return Math.Abs((t - obj).TotalSeconds) < 1;
        }

        public static bool ApproximatelyEqual(this DateTime? t, DateTime? obj)
        {
            if (!t.HasValue && !obj.HasValue)
            {
                return true;
            }

            if ((t.HasValue && !obj.HasValue) || !t.HasValue && obj.HasValue)
            {
                return false;
            }

            return Math.Abs((t.Value - obj.Value).TotalSeconds) < 1;
        }
    }
}