public sealed class IsDateAfter : ValidationAttribute, IClientValidatable
{
    private readonly string testedPropertyName;
    private readonly bool allowEqualDates;

    public IsDateAfter(string testedPropertyName, bool allowEqualDates = false)
    {
        this.testedPropertyName = testedPropertyName;
        this.allowEqualDates = allowEqualDates;
    }

    protected override ValidationResult IsValid(object value, ValidationContext validationContext)
    {
        var propertyTestedInfo = validationContext.ObjectType.GetProperty(this.testedPropertyName);
        if (propertyTestedInfo == null)
        {
            return new ValidationResult(string.Format("unknown property {0}", this.testedPropertyName));
        }

        var propertyTestedValue = propertyTestedInfo.GetValue(validationContext.ObjectInstance, null);

        if (value == null || !(value is DateTime))
        {
            return ValidationResult.Success;
        }

        if (propertyTestedValue == null || !(propertyTestedValue is DateTime))
        {
            return ValidationResult.Success;
        }

        // Compare values
        if ((DateTime)value >= (DateTime)propertyTestedValue)
        {
            if (this.allowEqualDates)
            {
                return ValidationResult.Success;
            }
            if ((DateTime)value > (DateTime)propertyTestedValue)
            {
                return ValidationResult.Success;
            }
        }

        return new ValidationResult(FormatErrorMessage(validationContext.DisplayName));
    }

    public IEnumerable<ModelClientValidationRule> GetClientValidationRules(ModelMetadata metadata, ControllerContext context)
    {
        var rule = new ModelClientValidationRule
        {
            ErrorMessage = this.ErrorMessageString,
            ValidationType = "isdateafter"
        };
        rule.ValidationParameters["propertytested"] = this.testedPropertyName;
        rule.ValidationParameters["allowequaldates"] = this.allowEqualDates;
        yield return rule;
    }

public virtual DateTime StartDate { get; set; }

[IsDateAfter("StartDate", true, ErrorMessage="End date needs to be after start date")]
public virtual DateTime EndDate { get; set; }

$.validator.unobtrusive.adapters.add(
    'isdateafter', ['propertytested', 'allowequaldates'], function (options) {
    options.rules['isdateafter'] = options.params;
    options.messages['isdateafter'] = options.message;
});
$.validator.addMethod("isdateafter", function(value, element, params) {
    alert(params.propertytested);
    var startdatevalue = $('input[name="' + params.propertytested + '"]').val();
    if (!value || !startdatevalue) return true;
    return (params.allowequaldates) ? Date.parse(startdatevalue) <= Date.parse(value) : Date.parse(startdatevalue) < Date.parse(value);
}, '');

public class TrainingDateEditViewModel
    {
        #region Properties

        /// <summary>
        /// Gets or sets CalendarEntry.
        /// </summary>
        public CalendarEntry CalendarEntry { get; set; }
....

<input type="text" value="" name="CalendarEntry.EndDate" id="CalendarEntry_EndDate" data-val-isdateafter-propertytested="StartDate" data-val-isdateafter-allowequaldates="True" data-val-isdateafter="End date needs to be after start date" data-val="true">

data-val-isdateafter-propertytested="StartDate" and IT SHOULD BE: "CalendarEntry.StartDate".

$.validator.addMethod("isdateafter", function(value, element, params) {
    var parts = element.name.split(".");
    var prefix = "";
    if (parts.length > 1)
        prefix = parts[0] + ".";
    var startdatevalue = $('input[name="' + prefix + params.propertytested + '"]').val();
    if (!value || !startdatevalue)
        return true;
    return (params.allowequaldates) ? Date.parse(startdatevalue) <= Date.parse(value) :
        Date.parse(startdatevalue) < Date.parse(value);
});