NUnit assertion for DateTime comparison

DateTimeIsApproximatelyEqualAssertion.cs

using System;
using NUnit.Framework.Constraints;

namespace Foundation.Testing.Assertions
{

    public sealed class DateTimeIsApproximatelyConstraint : Constraint
    {
        private readonly DateTime _upper;
        private readonly DateTime _lower;

        public DateTimeIsApproximatelyConstraint(DateTime expected, int toleranceInMs = 1000)
        {
            _lower = expected.AddMilliseconds(-toleranceInMs);
            _upper = expected.AddMilliseconds(toleranceInMs);

            this.Description = $" to be between {_lower} and {_upper}";
        }

        public override ConstraintResult ApplyTo<TActual>(TActual actual)
        {

            var actualAsDateTime = actual as DateTime?;

            var passes = actualAsDateTime != null && (actualAsDateTime.Value >= _lower && actualAsDateTime.Value <= _upper);

            return new ConstraintResult(this, actual, passes);
        }
    }

    public static class DateTimeIsApproximatelyConstraintExtensions
    {
        public static DateTimeIsApproximatelyConstraint Approximately(this ConstraintExpression expression, DateTime expected, int toleranceInMs = 1000)
        {
            var constraint = new DateTimeIsApproximatelyConstraint(expected, toleranceInMs);
            expression.Append(constraint);
            return constraint;
        }
    }

    public class Is : NUnit.Framework.Is
    {
        public static DateTimeIsApproximatelyConstraint Approximately(DateTime expected, int toleranceInMs = 1000)
        {
            return new DateTimeIsApproximatelyConstraint(expected, toleranceInMs);
        }
    }
}