enum Tag
{
    Name, Date, //There are many more
}

class RequestDictionary : Dictionary<Tag, string>
{
}

enum Tag
{
    Name, Date
}

interface ITags
{
    string Name { get; set; }
    DateTime Date { get; set; }
}

class RequestDictionary : Dictionary<Tag, string>, ITags
{
    public ITags Tags { get { return this; } }

    string ITags.Name
    {
        get { return this[Tag.Name]; }
        set { this[Tag.Name] = value; }
    }

    DateTime ITags.Date
    {
        get { return DateTime.ParseExact(this[Tag.Date], API_DATE_FORMAT, CultureInfo.InvariantCulture); }
        set { this[Tag.Name] = value.ToString(API_DATE_FORMAT); }
    }
}

dict[Tag.Date] = DateTime.Now.ToString(API_DATE_FORMAT);

dict.Tags.Date = DateTime.Now;

var dict = new RequestDictionary
{
    Tags.Date = DateTime.Now
}

var dict = new RequestDictionary
{
    { Tags.Date, DateTime.Now.ToString(API_DATE_FORMAT) }
};