Question: I need to learn how to use String.Format to insert colons in the middle of a 4 char strings, e.g. 1300 -> 13:00, or 0500 -> 05:00

Answer: 
You could use [`string.Insert`](http://msdn.microsoft.com/en-us/library/system.string.insert%28v=vs.110%29.aspx) method.

```
var formattedString = yourString.Insert(2, ":");

```
This code do the job:

```
string str = "1234";
str=str.Insert(2, ":");

 Console.Write(str);

```