DOS File DateTime
=================

https://www.ctyme.com/intr/rb-2992.htm

DOS 2+ - GET FILE'S LAST-WRITTEN DATE AND TIME
  INT 21H
    AX = 5700H
    BX = file handle

returns

CX = file time
  bits  0.. 4 (5 bits, 0.. 31): seconds/2
  bits  5..10 (6 bits, 0.. 63): minutes
  bits 11..15 (5 bits, 0.. 31): hours

DX = file date
  bits  0.. 4 (5 bits, 0.. 31): day
  bits  5.. 8 (4 bits, 0.. 15): month
  bits  9..15 (7 bits, 0..127): year - 1980



https://www.ctyme.com/intr/rb-2993.htm

DOS 2+ - SET FILE'S LAST-WRITTEN DATE AND TIME
  INT 21H
    AX = 5701H
    BX = file handle
    CX = new time
    DX = new date


handling of invalid values is not specified
as of chatgpt, mostly saved unchanged (and unchecked)



boot_plain_fat12_msdos.asm:

year  equ 2008
month equ 9
day equ 26
hour  equ 10
minute  equ 54
second  equ 49
; don't touch these definitions:
date  equ ((year-1980) << 9 ) + (month << 5) + day
time  equ (hour << 11) + (minute << 5 ) + (second >> 1)


