Machine learning or NLP approach to convert string about month ,year into dates
I got my answer , NLTK is good to go for this problem. 
You may use sutime with python wrapper : 

Python wrapper for Stanford CoreNLP's SUTime 

The usual approach in NLP is to collect a dataset required for training. Process that dataset so that the words in the dataset are converted into numbers.

One simple example of converting it into numbers is to make a large dictionary of words from the dataset and use the index of each word in the dictionary as the representing number