GN_Gmd, Get (read) the current machine system date in internal format

 IN:
     DE = 0, not allowed.
     DE = 1, not allowed.
     DE = 2, return system date in ABC.
     DE > 255, pointer to write machine date at (DE).

OUT:
     If DE(in)>255 then DE = DE(in) + 3; ABC = ABC(in)
     If DE(in)<255 then DE = DE(in); ABC = result

Registers changed after return:
     ......../IXIY same
     AFBCDEHL/.... different