INT 21h AH=2Ah returns the date DOS keeps: the year as a full number in CX (2026, not 26), the month in DH and the day in DL, all binary rather than BCD, and the day of the week in AL, counted from 0 for Sunday. DOS works out the weekday itself; it is ready for picking a name from a table.

prints the box's date, for example: 2026-10-23 Fri
MOV     AH, 2Ah       ; DOS: get the date
INT     21h           ; CX = year, DH = month, DL = day, AL = weekday
PUSH    AX            ; keep the weekday
PUSH    DX            ; and the month and day
MOV     AX, CX
CALL    PDEC1         ; the year
MOV     DL, "-"
MOV     AH, 02h       ; DOS: print the character in DL
INT     21h
POP     DX
PUSH    DX
MOV     AL, DH
CALL    P2D1          ; the month
MOV     DL, "-"
MOV     AH, 02h       ; DOS: print the character in DL
INT     21h
POP     DX
MOV     AL, DL
CALL    P2D1          ; the day
MOV     DL, " "
MOV     AH, 02h       ; DOS: print the character in DL
INT     21h
POP     AX            ; AL = the weekday, 0 = Sunday
MOV     AH, 3
MUL     AH            ; AX = weekday * 3: its name in the table
ADD     AX, OFFSET DAYS1
MOV     DX, AX
MOV     CX, 3         ; three letters
MOV     BX, 1         ; to standard output
MOV     AH, 40h       ; DOS: write to a handle
INT     21h
 
DAYS1   DB      "SunMonTueWedThuFriSat"
PDEC1   PROC    NEAR  ; print AX in decimal
MOV     BX, 10
XOR     CX, CX        ; digits so far
PD1:
XOR     DX, DX
DIV     BX            ; AX = AX / 10, DX = the last digit
PUSH    DX
INC     CX
TEST    AX, AX
JNZ     PD1
PD2:
POP     DX            ; the digits come back first to last
ADD     DL, "0"
MOV     AH, 02h
INT     21h
LOOP    PD2
RET
PDEC1   ENDP
P2D1    PROC    NEAR  ; print AL (0-99) as two digits
AAM                   ; AH = AL / 10, AL = AL mod 10
ADD     AX, 3030h     ; both digits to characters
PUSH    AX
MOV     DL, AH        ; the tens
MOV     AH, 02h
INT     21h
POP     DX            ; DL = the units
MOV     AH, 02h
INT     21h
RET
P2D1    ENDP

The year goes through a small decimal printer (divide by 10, push the digits, pop them in reverse), the month and day through AAM, which splits a value below 100 into its two decimal digits. The weekday's name is written with INT 21h AH=40h, which takes a length, since the table has no "$" between the names.

Setting the date

INT 21h AH=2Bh takes the date in the same registers. It checks them first: a month outside 1–12, a day the month does not have (30 February, 31 April, 29 February outside a leap year) or a year before 1980 gives AL = FFh and changes nothing; AL = 00h means the date is set. MS-DOS also refuses years after 2099. DOS 3.3 and later write the new date into the CMOS clock of an AT as well, so it survives a reboot.

prints: FF 00 (30 February refused, then today set to itself)
MOV     AH, 2Ah       ; DOS: get the date
INT     21h
PUSH    DX            ; today, for later
PUSH    CX
MOV     DX, 021Eh     ; DH = 2, DL = 30: 30 February
MOV     AH, 2Bh       ; DOS: set the date
INT     21h           ; AL = FFh: refused, nothing changed
CALL    HEX2
MOV     DL, " "
MOV     AH, 02h       ; DOS: print the character in DL
INT     21h
POP     CX            ; today again
POP     DX
MOV     AH, 2Bh
INT     21h           ; the date it already was: AL = 00h
CALL    HEX2
 
HEX2    PROC    NEAR  ; print AL as two hex digits
PUSH    AX
MOV     CL, 4
SHR     AL, CL        ; the high digit first
CALL    NIB1
POP     AX
AND     AL, 0Fh       ; then the low one
CALL    NIB1
RET
HEX2    ENDP
NIB1    PROC    NEAR  ; print AL (0-15) as one hex digit
ADD     AL, "0"
CMP     AL, "9"
JBE     NIB2
ADD     AL, 7         ; 10-15 become "A"-"F"
NIB2:
MOV     DL, AL
MOV     AH, 02h
INT     21h
RET
NIB1    ENDP

The example leaves the clock alone: it offers a date that does not exist, which is refused, and then sets today's date again. In DOSBox-X a successful set of the date or the time also switches off the "synchronize time" option, which keeps the clock in step with the host, if it was on.

Pitfalls

The date advances only while something reads the clock through DOS. The BIOS counts timer ticks and sets a "midnight passed" flag when the count wraps; DOS adds a day when it sees the flag. A program that reads the tick count with INT 1Ah AH=00h clears that flag itself, and DOS misses the change of day. The DOS date and the CMOS clock (INT 1Ah AH=04h) can therefore disagree: DOS reads the CMOS once at boot and counts on its own after that. A program reading the date and then the time just before midnight can get the old day with the new time; read the date again after the time and start over if it changed.