Data warehouse: using day number in Epoch in a date dimension
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.