doc: ADR for date override

lms/djangoapps/certificates/docs/decisions/006-cert-date-override.rst

+Certificate Date Override
+=========================
+
+Status
+------
+Accepted
+
+Background
+----------
+Generally speaking, a learner is awarded a course certificate upon earning a
+passing grade in a course. (Other ADRs go into the specifics of when a
+certificate is earned.) The data backing a course certificate is stored in the
+``GeneratedCertificate`` model. The course certificate generally displays an
+"Issued On" date. This date might be the associated course’s
+``certificate_available_date`` if set and elapsed; otherwise it will be the
+``GeneratedCertificate`` ``modified`` date.
+
+However, the ``modified`` date can be advanced by several backend processing
+systems. This means that a course certificate can display a confusing issue
+date. When this date changes to a date that is significantly distanced from the
+actual date that the learner earned the certificate, it can cause problems for
+the learner if they have an outcome (raise, promotion, acceptance to academic
+program) that is related to when they earned the certificate.
+
+
+Decision
+--------
+We will add a new model to the certificates app called
+``CertificateDateOverride``. This model will be accessed through the Django Admin
+and used to manually override the date on course certificates.
+
+We are choosing to make a new, separate model and associate it with
+``GeneratedCertificate`` (rather than adding new fields to ``GeneratedCertificate``)
+because we expect the number of certificates that will need to use this feature
+to be very low. We don’t want to add a new field to a potentially very large
+table that may only be used by a small number of records.
+``CertificateDateOverride`` will have a ``OneToOne`` Relationship with
+``GeneratedCertificate``; each certificate should have only one date override, and
+a date override will belong to only one certificate. The new table will have an
+accompanying history table.
+
+In addition to the date itself, we would like to keep track of some other pieces
+of information around the date: the ``user`` who made the date override, and the
+``reason`` why. While ``django-simple-history`` is able to provide us with these
+things, we decided to keep this data directly on the ``CertificateDateOverride``
+model to be able to surface them more directly in case the record is used in the
+product UI in the future.
+
+To edit or remove a date override, admin users should return to the relevant
+``CertificateDateOverride`` admin page and either edit the date or delete the
+record, respectively. This is a different approach than some analogous models in
+our app, where data is "removed" by toggling an ``active`` flag. In our case, we
+are comfortable truly deleting the ``CertificateDateOverride`` objects, and
+comfortable relying on the history table’s record of change.
+
+The ``CertificateDateOverride`` will override all other date logic when it comes
+to the date displayed on the certificate. (This includes overriding the
+``certificate_available_date``.) However, the ``CertificateDateOverride`` will not
+affect the availability or visibility of the certificate.
+
+Whenever a ``CertificateDateOverride`` is saved (either for the first time or
+after a change) or deleted, it will trigger the ``COURSE_CERT_CHANGED`` signal.
+The date override data (if present) will be included in the post to
+`Credentials`_ whenever this signal is called. Credentials will save and use
+this data for the Program Record page.
+
+.. _Credentials: https://github.com/edx/credentials