A set of dates obviously has an earliest date. The humour here is that an earliest date within the set of uninteresting dates is itself an interesting date by virtue of being the earliest such date in that set.
Uhh, no you’re thinking in terms of a finite set. From the set of all dates, what’s the latest possible date, or conversely the earliest possible date. The temptation is to say infinite BC is the earliest possible date, but infinity is not a number.
Infinity is not a number, it's a set of numbers, but specific infinities (e.g., the cardinality of the natural numbers, aleph-naught) are individual numbers, just not finite numbers.