Allow to represent a YYYY-MM-DD date stamp

Summary:
Azhàr defined global functions to convert between unixtime
and yyyy-mm-dd datestmaps.

This change creates instead a DateStamp class to represent
the date stamp.

Test Plan: Unit tests provided.

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D1635