Zoho Sign - expire_by date format

Zoho Sign - expire_by date format

Trying to write a function to extend the expiry date on Zoho Sign by 3 days. 

When reading data, the expiry date appears as follows:

 "expire_by": 1522866540000

Reading this and turning it in to a standard date format is fine.  How can I take a standard date and convert it in to this format?

Kind regards

Matt