Comparing two dates using one calendar table and USERELATIONSHIP

just to clarify: you state "two dates in a single measure"  and then

I have two tables:

1) Fact Sales ([Order ID], [Est Delivery Date ID], [Delivery Date ID])

2) Dim Date ([D Date ID], [D Date])

..... I've underlined the terms that are confusing me....

Is there sequentiality of the Dim Date ID such that you don't need the actual date itself but simply just their comparitive relative values i.e. So that you seek a record count of Table 1 where DD > EDD   ?