How do i cache data and update if its changes in database

Question

I have to display a date in my page which updated once in a month but there is not any fix day when it gets updated.

 

Since its updated only once in a month. I am planning to cache this date in client. 

 

But how do i update this chached date when there is new date?

 

How do i figure if date in cache is same or different?