Prompt user for DateTime
Note that I'm not very familiar with C#, so the code that I write may not actually compile.




  Let's say I need to get 20 dates. Is there an easier way to do this, like only having one prompt per date?


There's a much easier way to do this, and that is to extract this to it's own separate method. This method should do just what your code is doing now: prompt the user for the day, the month, and the year, and should return a DateTime.

public static DateTime PromptDateTime()
{
    Console.WriteLine("Day: ");
    var day = Convert.ToInt32(Console.ReadLine());

    Console.WriteLine("Month: ");
    var month = Convert.ToInt32(Console.ReadLine());

    ... year...

    return new DateTime(year, month, day, 0, 0, 0, 0);
}


Need to call this method twenty times? Go ahead (you'd probably want to use a loop though). With an extracted method like the one above, you won't have to worry about having cluttered code.



Validate Input

This is how you are getting input:


var dateDay = Console.ReadLine();
var dateDayInt = Convert.ToInt32(dateDay);



What if the user enters something like this?


  LOOK MA! NO VALIDATION


What's your code going to do? It's going to throw an error because Convert.ToInt32 doesn't quite know how to parse that.

To validate the user's input, you should use int.TryParse. This method takes a string and an output variable and attempts to parse the string into a number and puts the parsed number in the output variable. If the parse is unsuccessful, then this method returns false.

What this method could look like in action:

var input = Console.ReadLine();
var num;
if(int.TryParse(input, out num))
{
    Valid!
} else
{
   Invalid!
}


Of course, this is a little much to put inside this method you have (considering the fact that you'll have to keep looping to get the user's proper input). Therefore, you should extract this into a method.

public static int ReadInteger() {
    while(true) {
        var input = Console.ReadLine();
        var num;
        if(int.TryParse(input, out num))
        {
            return num;
        }
    }
}


The above method keeps on looping until proper input is receive. As you may notice, this is not very suitable for unit testing as STDIN is forced. However, as Dan Lyons commented:


  Console.WriteLine utilizes Console.Out for everything - if the method took in a TextWriter, he could use Console.Out in "production", but a StringWriter in test.


Now, with proper validation, your extracted method would look like this:

public static DateTime PromptDateTime()
{
    Console.WriteLine("Day: ");
    var day = ReadInteger();

    Console.WriteLine("Month: ");
    var month = ReadInteger();

    ... year...

    return new DateTime(year, month, day, 0, 0, 0, 0);
}




As svick mentioned, this is still technically not enough validation. What if, for the month, the user enters


  13


That doesn't make much sense, does it?

This, of course, can lead to some complications. Some months had 30 days and others have 31. Validation in general should be mostly simple; you just check that the month number is between 1 and 12 (or 0 and 11, but be consistent), and that the year number is between 1 and the current year (unless you want to handle BC/BCE, in which you'd have to do some extra checking).

However, this still doesn't solve the days-in-month problem. A solution could be to create a map/table showing the months and the corresponding amount of days and use that to look up whether the day number is valid. However, then you encounter February (yay!) with which you have to do further validation. Honestly, your best bet would be to go with Zack's solution as it is more idiomatic and would probably be easier to do parsing with (although, I don't know how it handles that days-in-month issue either).