What is the date format of the Telephony.Sms.Inbox.DATE_SENT column return string?

While retrieving SMS info from the Telephony content provider, I came across an unfamiliar time format for the time/date in which the messages were sent or received.

Example:

date_received: 1431112280847 (Message receive May 8, 2015 at around 12 pm)

I have never seen this date format before. Any help will be much appreciated.