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, ":");

```
What about the String.Insert method 
[http://msdn.microsoft.com/en-us/library/system.string.insert(v=vs.110).aspx](http://msdn.microsoft.com/en-us/library/system.string.insert%28v=vs.110%29.aspx)
I hope that's help :)