RtcDateTime vs year below 2000

[Elmaranin]

Why all these new modern libraries use new format 'since 2000 year'? Most of all old hardware still uses unix-time format 'since 1970..'.

test code that fails:
RtcDateTime test = RtcDateTime(1970, 1, 1, 1, 1, 1);

in RtcDateTime.h:

RtcDateTime(uint16_t year,
uint8_t month,
uint8_t dayOfMonth,
uint8_t hour,
uint8_t minute,
uint8_t second) :
_yearFrom2000((year >= c_OriginYear) ? year - c_OriginYear : year),

how it can convert here
_yearFrom2000((year >= c_OriginYear) ? year - c_OriginYear : year),
1970 for ex, to uint8_t type?
And how can this library at all work correct with data until 2000?
My DS3231 battery is down, when time is readed it correctly returns 1970... zero unix-time, but this RtcDateTime library can't correctly give it to me.
And code in 'DS3231_Simple.ino' fails too in this case:
RtcDateTime now = Rtc.GetDateTime();
if (now < compiled)

[Makuna]

This is by design. These are meant to track current time from a hardware Real Time Clock.

The DS3231 hardware itself cannot even support years before 2000. It stores it as a two digit year offset from 2000. So I have no idea where you get support for 1970 from.