INT 1A - TIME - SET REAL-TIME CLOCK TIME (AT,XT286,PS)
AH = 03h
CH = hour (BCD)
CL = minutes (BCD)
DH = seconds (BCD)
DL = daylight savings flag (00h standard time, 01h daylight time)

See also: 1A01

1A03 - TIME - SET REAL-TIME CLOCK TIME (AT,XT286,PS)