How to create a includes folder inside sites/all/module/custom/ I want to create a folder name include in-side sites/all/custom/contrib/  and  inside this want to write some .inc files like date.inc, string.inc in each file a class is created which holds function. 

Now I want to use this functions in each module under sites/all/custom/contrib. I have 10 custom module so this .inc functions are available to each custom module.  

What is the directory structure I have to follow and how can I achieve this?
In one of your custom modules, just create a directory includes and place your .inc files inside it.

To use the .inc files in your .module file, you can use module_load_include.