Dealing with dates becomes much simpler conceptually when you realize that you are actually dealing with two different ideas masquerading as one: dates, and the actual instants in time which they name. Instants in time can only be unambiguously referenced by measuring some fixed unit of time intervals from some commonly agreed upon event (e.g. seconds since the Unix Epoch).
Dates are a mapping from human-readable labels to instants in time; critically, this mapping is not a total function (there are dates that map to no instant in time in a particular calendar, such as October 5-14 1582 in the Gregorian calendar, or 2:30am March 8 2026 in US timezones due to DST), nor is it in fact a function at all depending on how you define your inputs (1:30am November 1 2026 in US timezones is ambiguous if you don't specify DST/ST).
Furthermore, the mappings (both for past dates and future dates!) can themselves change over time (changes to DST, timezones, and calendars such as in TFA), and you need to keep track of these changes in perpetuity, as well as at which instant in time they were implemented, and we don't necessarily have that information for some of these historical changes, we only have the date as written down, which itself can suffer from the ambiguity problem (is it the date pre- or post- calendar change?).
This makes dealing with past dates tricky, and dealing with future dates speculative at best. For instance, what's February 28th + 1 day? The answer depends on the calendar system and the year; we can answer it definitively for past years but not necessarily for future years (maybe in 2040 we'll move the leap day to December 32nd). This is why it's important to immediately convert past dates (and sometimes future dates, depending on the use case) to unambiguous instants in time, and only convert them back to dates for display purposes.
"Wednesday, August 12th 2026" and "8 days from now" are also semantically distinct for some purposes: if have 8 doses of a daily prescription left, I'm still going to run out in 8 days even if tomorrow the government decides to repeal the Gregorian calendar, thus changing the name of "8 days from now" from "Wednesday, August 12th 2026" to "Sunday, August 2nd 2026". My medication doesn't care what the government decides to call 8 days from now, I'm going to run out either way. The pharmacy, on the other hand, very much does care what the government decides to call 8 days from now, because perhaps they're closed on Sundays. The calendar change means that "8 days from now" went from being not-a-Sunday to a Sunday, affecting their operating hours.
It's really just another instance of one of the two hard problems in computer science: naming things.