Epoch - how is it meant?

3.3.3 Date and DateTime reads in part:

"A Date is a subtype of number; the number is the number of days from a particular date called the epoch. Thus, a date when presented as a general-purpose number is also called a serial number. This specification does not specify the exact value of the epoch, but implementations shall support all dates from 1904-01-01 through 9999-12-31 (inclusive), with correct calculations."

That is two different uses of "epoch."

One use is to designate a start date, so that other dates can be specified unambiguously. Fixed only when declared.

The other use is in the final sentence, which appears to refer to epoch as some outside designation that is not fixed, but requires a range of dates to be supported.

The "thus" of the second sentence appears to be misused. I don't see how the second sentence follows from the first.

For the moment I am would drop the second sentence. "...called a serial number?" Must mean serial date?