DateTime.Compare

DateTime.Compare

var stringDate = "20190130";
            
var startDate = new DateTime(2019, 01, 30);
var endDate = new DateTime(2019, 02, 20);
            
var testDate = new DateTime(2019, 02, 01);

var result1 = DateTime.Compare(testDate, startDate);
var result2 = DateTime.Compare(testDate, endDate);
            
Console.WriteLine(result1); // 1
Console.WriteLine(result2); // -1

if (DateTime.Compare(testDate, startDate) >= 0 && DateTime.Compare(testDate, endDate) <= 0)
{
    Console.WriteLine("It is inside");
}
else
{
    Console.WriteLine("It is outside");
}