Data warehouse: using day number in Epoch in a date dimension In Chapter 2 Retail Sales of Ralph Kimball's The Data Warehouse Toolkit, the author describes a date dimension with three fields which appear totally alien to me: 

Date Key (PK)
Date
Full Date Description
Day of Week
**Day Number in Epoch**
**Week Number in Epoch**
**Month Number in Epoch**
Day Number in Calendar Month
Day Number in Calendar Year
...


I understand that an Epoch is the "instance in time chosen as a particular era" and that the current standard epoch for J2000.0 starts on January 1, 2000. But I don't see how these numbers would be helpful in a date dimension or a cube used for getting information about retail sales.  


Are numbers in Epoch typically used in Date Dimensions? 
What Epoch would be used for a modern day business?
How would a user slice data based on Epoch and why?
Epoch is also a synonym for a date or time. Occasionally used in database models as an alternative to the ubiquitous 'timestamp' as a column name, the two are essentially interchangeable.

Edit: Looking at your list with fresh eyes (and without getting the book from the cupboard it's hiding in) I expect an epoch in this instance would be a period such as a financial quarter. For example, if a the quarter ran from 1st April to 30th June, 'Day number in Epoch' for 1st May would be 32.