How to convert date format such as 7/18/2015 to the number of month

How to convert date format such as 7/18/2015 to the number of month from requesting date 'date' in sh scripting ?

Let say I have output in my log.txt -> 7/18/2015. How I convert it to the full number of month starting from 'date' till 7/18/2015 in shell scripting ? Thanks in advance.

we are clear little bit ,Can you post the output you want?

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Typeo error we are not clear