Custom Time Validation by Data annotation in MVC

I am trying to implement custom validation in my MVC application. My scenario is

If Closed checkbox is checked, Start and End times are not required.

If Closed checkbox is not checked, I want user to select start and End times from dropdown.

If user select start and end times, I want to validate end time should be greater than start time.

I wrote a custom validation class as below. How can I validate this ?

 public class DatesValidation : ValidationAttribute
{
    public bool Closed { get; set; }

    public string Start { get; set; }

    public string End { get; set; }

    public DatesValidation(bool closed, string startTime, string endTime)
    {
        this.Closed = closed;
        this.Start = startTime;
        this.End = endTime;
    }
}