# C#, <strike>128</strike> 127 Bytes

    using System;class P{static void Main(){for(;;)Console.Write($"\r{new DateTime(2019,3,31)-DateTime.Now:ddd\\:hh\\:mm\\:ss}");}}

Ungolfed code:

    using System;
    class P
    {
        static void Main()
        {
            for(;;)
                Console.Write($"\r{new DateTime(2019,3,31)-DateTime.Now:ddd\\:hh\\:mm\\:ss}"); 
        }
    }

I would not have figured out the \r trick without help from the other C# answers here.

For anyone looking for further improvement, you can also put the Write() expression inside the for loop. Seems like I should be able to save a byte somehow here, because that saves me the semi-colon for that statement, but it works out to the same number because you can't have a fully empty body:

    using System;class P{static void Main(){for(;;Console.Write($"\r{new DateTime(2019,3,31)-DateTime.Now:ddd\\:hh\\:mm\\:ss}"));}}

  [1]: http://codegolf.stackexchange.com/a/115049/58322