Classifying dates in sentences
If you have enough data to train a machine learning model, I believe LSTMs are what you need. Long short-term memory (LSTM) is an artificial recurrent neural network (RNN) architecture used in the field of deep learning. 

LSTM networks are well-suited to classifying, processing and making predictions based on time series data, since there can be lags of unknown duration between important events in a time series.

Initially, you can use Pandas LSTM library, you design it from scratch by using Python Tensorflow library. Look at the following links:

Wikipedia

LSTM in python tutorial