ML> I surmise (JdeBP probably knows) that the OS/2 CLOCK$ drivers read 
 ML> only the two low-order digits of the RTC clock year, and effectively 
 ML> put the two upper digits into the CMOS "century byte" by using 
 ML> windowing:  Any year shown as greater than 79 gets 19 in that byte; 
 ML> any year less than 80 gets 20.  

Excerpted from the `calc_year_sec' routine in CLOCK01.ASM off the OS/2 Warp 4
DDK:

------------------------------------------------------------------------
DATEOK:
; if year is over 20 centry ( year < 80 )
;    year += 100
        XOR     AH,AH                   ; AX = LOWER PART OF YEAR
        CMP     AL,80                   ; Is it 20xx?
        JGE     calc20                  ; No, Add only 1900
        ADD     AX,100                  ; Yes, add extra 100
; year += 1900
; save on stack
calc20: ADD     AX,1900                 ; AX = Year and century
        MOV     century_year,ax         ;
        SUB     AX,1980                 ; Get years since 1980
------------------------------------------------------------------------

This routine is called from `FIXISEG', which is the routine that reads from
the RTC registers and writes into the (16-bit) global infoseg.

 ¯ JdeBP ®

--- FleetStreet 1.22 NR