An anchor date is a single time value which anchors the validity of a piece of data to that particular time. In other words, it is a time when the validity of data is either known or projected to be true.

Anchor dates cannot be entered for values earlier than the start date for a data value (they are not allowed for projections into the future, i.e. where the start date is later than the input date).

When no start date exists, the anchor date serves as a confirmation that that data was valid AS OF the anchor date, and presumably for some unknown time before as well. Multiple anchor dates may be stored for the same data value, serving as evidence of validity over a length of time.

An anchor date describes validity at a particular time; it does not by itself establish when validity began. The reference to projected validity is subject to the date restrictions above and does not waive them.